Even if they could run it playing on a PC means having to deal with the mess of using a computer. While some of us like that, most people do not. Hence why computer companies are always trying to pass things off as easier, nicer, better user experience. Because for a lot of people the experience of using a computer is like dental surgery. The fact that so many PC games ship in a horrible state or you're waiting on driver updates from your video card company does not make this situation any better.
3. XBOX Live and PSN are actually good things. I know this confusing the crap out of a lot of PC gamers, but they are great. For friends, leader boards, trophies, and all the "social" aspects of gaming, people like them.
It appears you haven't gamed on computers for close to 10 years. Every now and then a new release will be bad (Dead Island) but the majority of games are perfectly fine at launch.
XBOX Live and PSN are crap. The lack of dedicated servers and charging people to play online are not and will never be good things. You know what PC users have? Steam friends list. Right click and join game with your friends.
This is way off topic from speculation about Xbox 720 hardware...