Most people are courteous, and extend their legs below the seat in front of them (which lowers their knees) so as to not impede the proper reclining of the seat in front of them.
Some people on the other hand expect the world to cater to them and therefore are idiots.
Some people on the other hand expect the world to cater to them and therefore are idiots.
I'm 6'5" and my knees are hard against the upright seat back. When I try to put my legs under the seat like you suggest, my shins hit the seat bottom before they extend at all. Net gains of zero. So I guess that makes me an idiot, huh?