It's as simple as that. But there is a purely financial aspect to it, as well. I was once talking to a CTO of a very large website who explained that they had done a cost analysis of their bandwidth. They found that with the amount of traffic they had, they were spending 16 cents per month per character in their HTML source code. So it made good financial sense to pare down as much as they possibly could.
Speed is the most important factor in the success of any site.
Make a slow site, and people won't use it. It's as simple as that. The problem with these sites, of course, is that they fail to account for what their audience is actually after when they visit. And that's a recipe for disaster, because once you've alienated your audience it's nearly impossible to get them back. I take development teams through a process of intimately understanding who their users are.