Because there was no need to.
We might guess that the attack occured very close to the hospital wing, since McGonagall and Dumbledore quickly thought that Colin wanted to visit Harry: "Another attack," said Dumbledore.
"
Minerva found him on the stairs."
"There was a bunch of grapes next to him," said Professor McGonagall.
"We think he was trying to sneak up here to visit Potter."
What is more, they might have not had their wands with them.
Dumbledore claims he was on the way to the kitchen for something to drink, he didn't need a wand for that: "
Yes," said Professor McGonagall.
"But I shudder to think...
If Albus hadn't been on the way downstairs for hot chocolate - who knows what might have--
" Finally, some pure speculation.
Maybe Dumbledore insisted on not using magic for the traces not to be erased.
We learn in the Half Blood Prince that every use of magic leaves some trail.
Dumbledore was standing in the middle of the cave, his wand held high as he turned slowly on the spot, examining the walls and ceiling.
"
Yes, this is the place," said Dumbledore.
"How can you tell?"
Harry spoke in a whisper.
"It has known magic," said Dumbledore simply.
It is possible that Dumbledore wanted to return to the stairway that night and further investigate the place, looking for some clues.
scifi.stackexchange.com/users/79379/Demosthenes
Dumbledore might be old, but he certainly isn't physically infirm.
Someone capable of swimming a hundred yards in "icy water" would certainly have the strength to lift and carry a small boy several tens of feet.
‘Then take off your Invisibility Cloak – there is no need for it now – and let us take the plunge.’
And with the sudden agility of a much younger man, Dumbledore slidfrom the boulder, landed in the sea and began to swim, with a perfectbreaststroke, towards the dark slit in the rock face, his lit wandheld in his teeth.
Harry pulled off his Cloak, stuffed it into hispocket and followed. Harry Potter and the Half-Blood Prince
scifi.stackexchange.com/users/20774/Valorum
First of all, we know that human levitation is possible and known.
In The Prizoner of Azkaban, we see Sirius Black use Mobilicorpus on Snape's unconcious body to levitate it away from the Shrieking Shack, bumping his head intentionally along the way.
It is safe to assume that between Dumbledore and McGonagall, one of them should have known the spell, though we (the audience) had never seen it at that point in the series.
Getting back into the tunnel was difficult.
Lupin, Pettigrew and Ronhad to turn sideways to manage it; Lupin still had Pettigrew coveredwith his wand.
Harry could see them edging awkwardly along the tunnelin single file.
Crookshanks was still in the lead.
Harry went rightafter Sirius, who was still making Snape drift along ahead of them; hekept bumping his lolling head on the low ceiling.
Harry had theimpression Sirius was making no effort to prevent this.
Harry Potter and the Prisoner of Azkaban Second, the source and nature of Colin's curse was unknown, and so they would have likely sought to err on the side of caution.
Any magic performed on him could have caused further damage to him , or to the caster.
Since the Hospital Wing was so close, they would have wanted to get Colin into Madame Pomfrey's expert hands as soon as they could without exacerbating the situation.
My third thought is a bit more speculative, but keeping their wands free (not being used on a continuous spell) means that they would have been able to defend themselves against whatever attacked Colin .
This argument is a bit more difficult to defend as their hands would have been occupied, but wand in hand while carrying the boy would have still afforded them the ability to cast a shield charm.
Casting Mobilicorpus could have made their reactions a tad slower.
Lastly, in situations of high stress, many wizards resort to muggle practices .
Harry Potter and George Weasley get in a fist fight ( muggle dueling ) with Draco Malfoy after a quidditch match when Draco insulted both of their families.
It is possible that the situation with Colin Creevey was dire enough that thought of spell casting was forgotten in the need to rush him to Hospital Wing.
scifi.stackexchange.com/users/78411/BlackThorn
Compassion and respect Expanding on my earlier comment, and to draw from other answers, we can all agree that Dumbledore and McGonagal are arguably two of the most magically gifted wizards at Hogwarts.
Both fully capable of performing magic that would render having to lift a child a simple task.
However, they are also teachers .
They have a responsibility for the safety and health of the students, and would be expected to care for one who is ill, injured of unhappy.
Yes, they could have levitated Colin, but IMO if they knew he was safe to hold, then they absolutely would have carried him because of their duty of care as his teacher and responsible guardian.
scifi.stackexchange.com/users/22980/Longshanks
In addition to the available excellent answers, there are two more points I would like raised:
First, it's a matter of respect for someone affected by an unknown (at the time) effect who may not come back from it.
We know that to a degree doing things "the muggle way" is used as such, as when the Weasleys do their chores that could easily be solved with magic.
Valorum in the comments has another excellent example of this: "Harry refuses to use magic to dig Dobby's grave", which in particular is at quite analogous to the scene in question.
Secondly, as the magical effect is in fact unknown there is no telling how it will interact with other magic.
This treatment is a precaution more than anything else.
scifi.stackexchange.com/users/70850/Weckar E.
Could I just point out that it's probably a bit of a surprise to anyone if they find anyone unconscious (or petrified) on a normal evening.
Not everyone is always 100% logical.
I mean, if I found an unconscious person in a warehouse I probably wouldn't think "I'll load him onto that handcart over there".
Especially if, as everyone else points out, he was probably very close to the infirmary already.
scifi.stackexchange.com/users/79449/Dagon
When Colin Creevey gets petrified in  The Chamber Of Secrets , Harry sees Dumbledore and McGonagall bringing him in the hospital wing.  He describes them as if they were carrying a statue.  Why would Dumbledore and McGonagall, pretty old (and thus physically weak) but pretty skilled wizards, resort to the inconvenience of carrying things like petrified people by hand instead of using magic to lift them?  Using magic would seem both safer for the carrier and for the carried.
Why would Dumbledore and McGonagall carry Colin Creevey with their arms?
scifi.stackexchange.com
<harry-potter><albus-dumbledore><minerva-mcgonagall>
scifi.stackexchange.com/questions/154031
scifi.stackexchange.com/users/54393/Fatalize
There is no clear answer to this question, but there are some potential reasons why this may have happened. Firstly, there was no need to use magic and not everyone always behaves in a logical way in emergency situations. Secondly, although Dumbledore is old, he is not necessary infirm and is perfectly capable of carrying someone to the infirmary. Finally, it's quite possible that they were only travelling a short distance, so there was no inconvenience in physically carrying Colin Creevey. Age was not an issue as we know Dumbledore was physically strong, and perhaps the use of magic would have been risky at that time, either in terms of making them more vulnerable and easily detectable or harming Harry.
There was no need to use magic as the attack occurred very close to the hospital and anyway magic might have left detectable traces. Dumbledore is physically strong despite being old - as we see when he swims a hundred yards in icy water. So no problem to carry Harry. There are scenes where levitation is used so it would certainly have been an option, but maybe they were afraid of being more vulnerable in the middle of performing the trick. Perhaps they were also too stressed to think of a magic solution and so did it the muggle way. They did it out of compassion and respect and also the responsibility they felt for the safety and health of their students. There are a number of times when muggle ways are used - e.g. when Harry is burying Dobby or the Weasleys are washing up. Also, the magic might have had side effects. The decision to carry Harry could just be an example of irrational behaviour - no-one acts logically 100% of the time.