I think this, and all the other weirdness, is because usage of thread-local variables is pretty low. Probably somebody wanted to have "lightweight" threads doing work as quickly as possible accessing only local variables.

Edit: ah, see https://www.akkadia.org/drepper/tls.pdf which I was clued into by https://news.ycombinator.com/item?id=43079061

The situation is much more complicated than it sounds, because the question of where the variables are isn't known at compile time. It has to be done at link time. Which may include dynamic link time as a library is loaded. That all sounds fairly horrible.

All to handle static constructors. If you don't have thread-static variables, or they don't have constructors, it's much simpler.