why do websites eg yahoo7 and yahoo answers change their appearance?

[Storkum]

[Alanis]

Some people will whine until they are hoarse, saying "wahhh no change just for the sake of change!", but in reality, it is for money.

Yahoo Mail probably removed its tabs to fit more room for ad space.
In the case of Y!A, the new Yahoo Answers looks a lot like what a social networking site would look, and that was done intentionally to draw more users from social networking sites. More users looking at ads on here means more money for Yahoo.