If I could flip a switch and change this one aspect of modern life, I'd be willing to re-learn all my old reading/writing habits. It just makes more sense to "build" ideas upward.
It's top down for same reason things are generally left to righ. In the days of ink and paper, you'd end up with a smeared mess - ask a lefty what a pain in the ass that is.
Less of an issue nowadays of course. Still the very idea is a bit nails on a chalkboard to me, but I'm old and wondering if it's a generational thing. Maybe you feel like this since many interfaces push text entry to the bottom?