I use Gmail because I don't care about my emails. Google can know that I'm subscribed to the Haskell, FSF and school mailing lists and that I sometimes talk about school projects or work, all without affecting me much. And that is all I use email for.

Now, I could run an email server myself, but I do not have the time, experience or inclination to do this. I could also use a provider that does not use proprietary software, but that would be pointless: since they're running the server, I would not have significantly more freedom than I do with Gmail.

So really, the reason is simple: it doesn't matter much from a practical or ideological standpoint, and I'm incredibly lazy.