Why do websites constantly redesign their site?

Inspired by the new Gmail. Why do companies constantly redesign their sites? Facebook seems intent on doing this once every 6 months (and no one ever likes it), Twitter, now Gmail, Livejournal, TV.com… I understand making it better, but many times, they actually have a lot of graphic designers writing in and saying “um, why would you take a decent design, or a great and pioneering design, and ruin it?”. And since it costs money to redesign a website, it’s probably not “because… we can??”. So, is there a reason that websites feel the need to redo the design so frequently, even if it drives away users?