The same reason Microsoft makes Office available for Mac. There's even an excellent first-party Messenger client for iOS too - Microsoft's objective is to maximise marketshare, not to make out little exclusives for itself. I hardly think anyone bought into WP7 because of Xbox integration. I think you'll find that the expression is "nobody cares" - gamerscore and XBL avatars are just numbers in a computer that have no bearing in real-life. They're manufactured things invented to make you think the wasted time spent getting teabagged online by foul-mouthed 13 year-old boys was somehow worthwhile.
I was disappointed iOS 5 introduced Twitter integration - because it means that Mobile Safari (and other apps) have huge "Tweet" buttons in menus that I can't remove, despite the fact I don't have a Twitter account.
If I get a WP7 device (wake me up when 128GB is available) I'm going to customise my home screen to have the following:
- Upcoming Calendar Events
- Unread email, SMS, and IM
- Nothing else.
Because nothing else matters in a phone.