The first consumer processors with multiple cores came out around 2005. Before there were multiple cores in consumer machines, there was no reason to implement concurrency in a way that would scale to multiple cores.
Not to mention, back when the Emacs we know today was created, there weren't many GUI platforms available in the first place. First public release of GNU Emacs - the one with Emacs Lisp - was in 1985, so the Emacs we know is ~3 years older than X11. Emacs added GUI support in 1986[0] - before X11 was a thing (though X itself existed since 1984).
Emacs started as a terminal app, the GUI was added as an afterthought, by pretending it's a TTY. The concept of a "UI thread" wasn't on Stallman's mind back then. It continued to evolve from there; fast forward 35 years, and now we're living with a GUI program that still thinks it's writing to a teletype[1].
