A couple of reasons besides stylistic pretension (though that might be true in some cases):

1. Skinny columns of text are easier to scan quickly. This is the same reason newspapers do it.

2. When you're using a fixed-width layout, you're going to look skinny at some screen resolutions and scroll horizontally on others. People are generally more horrified by the latter than the former, so they go conservative and target a fairly small screen size — in that case, 640px wide. Very likely, the people behind the site have a smaller display than you do (though a site targeting 640x480 is still probably aiming a few notches below their own display).