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.
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.
You lose, so what do I win? Seriously, you overestimate the value or accuracy of a facebook profile.
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.