You'd think so, but I've seen a lot of websites that dump a sticky "social sidebar" on top of the content, and if it's absolutely positioned you can't read the stuff that it's stuck in front of.

You are correct.

However, the sites that do the overkill of a sticky header tend to also have poorly thought out site navigation and user experiences.

Let's say on a normal site, killing any absolute nav would result in a 5%* decrease in UX.

On a site like this, killing any absolute nav might result in a 5% increase in UX.

But that doesn't work as well, since for example on this page there is JS event tied to scrolling the page that moves the header with you. So you'd have to tie your own anti-move trigger to the same event, which might mean same script won't be universal. If you just remove the header with a script, you can still get to the navigation by refreshing the page.