It's the DRM, of course. Just like many, many people on Amazon.com:
Spore is now apparently the most pirated game ever. The Washington Post has a nice article explaining the issue. Loyd Case gives his personal view, based on his long experience with PC gaming, here. (Thanks Mark for both articles.)
Spore is joining a group of games that I'd like to play, but won't. You know, like Rock Band. I hear there's a sequel.
(Ah, a friend of mine bought it online on Sunday and finished installing it on Tuesday...)