Because I'd rather play my xbox. It's much more simple to play my games with friends on xbox than pc. A lot of people dont even have pc's strong enough to run simple games. So for 300 bucks (or whatever they cost), plus online costs, I can play my games with my friends.
Also, if you think Halo and Forza are all that XBox has to offer then that's on you. It's a dope system, that's why so many people own and actively play it. If you don't like it
But remember, just because you don't like it doesn't mean it sucks.
My PS3 collects serious dust. My xbox gets about an hour a day of spin.