Something to remember about <PERSON> is he's a man of focus... har har...
But in that focus you may have to figure he has his reasons for making such choices. You mention the knife fighting scene, but didn't question why he chose to assemble a specific revolver and load it with one bullet. Weird choice, right? Especially since he has thus far disarmed everyone trying to get him in close quarters.
Numerous points in the films he does secure weapons from others as he needs them. He appears confident in his ability to secure the next weapon, so for the purpose of the film and his own boogie man behavior, having him ignore the fallen weapons of others seems rather inconsequential.
Other things to consider -
* Nobody in the <PERSON> world seems to have heard of a sniper rifle except <PERSON> character.
* Nobody seems to aim for his head
* Much of the villains in this world have a "Dr. Evil" complex about them and decide to "kill" <PERSON> in ways that leave so many options for escape it makes you wonder how they ever became crime lords in the first place (think plastic bag in the church)
* <PERSON> is wearing a magical suit that causes kinetic energy to just disappear.
* Nobody would have their sense of hearing left if this was the kind of thing they did day after day.
* Pills seem to prevent him from feeling pain in full force and seems to prevent bones from breaking. 516
(by this point every rib he has should be dust).
As just a few examples showing that the <PERSON> universe is not one to compare to the one we live in now. His is more like a video game, and probably is best to allow it to be that way. Scrutiny in this universe would work best as a drinking game that would land you in a hospital before the first half of the movie is over.
So... Why does he ignore them? Because he's a world class assassin who can turn hot air into a lethal weapon. He ignores dropped weapons because he deemed it so. In the <PERSON> world, that seems to suffice in many applications. 190