Question: What I mean is that is it more dangerous than other contact sports that aren't martial arts? Such as Football, Soccer, Basketball, etc...

And if yes, why?

Answer: 
I'm not sure that your statement about the safety of boxing is generally accepted.

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> ["There is absolutely no way you can make boxing safe," said Nelson Richards, MD, a delegate from the American Academy of Neurology who proposed the original resolution to ban the sport in 1983.](https://www.ama-assn.org/amednews/2002/07/08/hlsb0708.htm)
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[The BBC reported](http://www.bbc.co.uk/health/physical_health/conditions/boxing.shtml)

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> According to brain surgeons, over 80 per cent of professional boxers have serious brain scarring on MRI scans. The evidence for harm or cumulative brain damage to amateur boxers is less clear.
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Boxing advocates point out that amateur boxing [has fewer injuries](http://www.boxinggyms.com/gladiator/newpaper/safe.htm) than soccer, gymnastics, etc. However that source doesn't cite how they measure "fewer injuries", and doesn't state whether they count long term damage to the brain. [There is some evidence that even amateur boxing can cause brain damage.](http://www.science20.com/news_articles/even_amateur_boxing_can_cause_brain_damage-89512)

Ultimately, you may want to look at a source which [compares injury rates](http://www.thecni.org/reviews/11-1-p35-gerberkozora.htm). A quick google search suggests that [football and soccer](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2267335/) have the highest injury rate/hour practiced. But that doesn't address the severity or long term consequences of those injuries.
At a good gym, meaning experienced coaches and decent equipment, boxing/kick-boxing should not be that dangerous. First of all, you're probably not sparring right away, and once you are its in a controlled environment with mouthpieces, headgear, gloves, and shinpads(if kickboxing).

As pointed out in a previous answer, you are probably at an increased risk of facial bruises, bloody noses, etc, but not serious injury. However, if you are training for an MMA style of fighting which includes takedowns, your rate of injury is going to spike sharply.

Comparing it to other sports is tricky. Even at a high-school level we certainly had a higher level of general injuries in football, and typically more severe...broken bones not being uncommon. Basketball didn't have the same high incidence of really violent injuries, but a much higher incidence of high ankle sprains and the like. Soccer seemed relatively safe but I never played at a highly competitive level, and if you watch the Europeans play you'd think it was more dangerous than trying to snuff volcanoes with your bare hands with the frequency they go down screaming in pain.

In short, I think there are too many variables to objectively answer your question, but the above has been my (anecdotal) experience.