Because it's too hard to read posts when you have to slide the scrollbar back and forth all the time. Maybe some people with low resolution are used to it, but most of us aren't. And I don't complain too much, only when someone's signature is too long. I don't see any reason why people have to have astronomically long signatures that cause that cause the screen to scroll. They don't look good that way, anyway.
Everything changes..changes..
Things are changing constantly...
Everything changes...changes...It's evolutionary.
Everything changes...changes..
Changing all the time, playing with your mind
Modified or rearranged, everything has got to change