For about 175,000,000 reasons.
I'm afraid they control your web identity. You know what though, I'm okay with it. It makes my life easier.
I do have a Facebook profile... but I visit the site maybe once every three months, and all the data is the fake data I put in 4 years ago. My real identity is this thing called a "web page" that is the first Google result for my name.
The benefits of a coherent identity on and offline are only going to get more compelling.
You lose, so what do I win? Seriously, you overestimate the value or accuracy of a facebook profile.
Instead, I think we'll see social networking layer across websites generally: imagine using opensocial/oauth over multiple sites that you use, as if you could bolt Ning.com over the top of you blog or forum or startup site.
I tested out adding Google's friend connect on my wordpress personal blog. Fairly simple, but they still collect the data :( I guess a duopoly is better than a monopoly, but there's still room for web identity to develop outside of Facebook.
Facebook could replace dating, restaurant review, business productivity, classified sites, and expand into things we haven't thought of yet, but that requires a substantial user base. The company is still growing out of its college foundation, but judging by its growing acceptance among the 25+ age group, it could start gaining value fast.