macOS still has a completely retarded implementation of DisplayPort MST (i.e. the only supported display is their own 6K display).

It's been growing on me for years, as macOS is my favourite platform, but I could go on a full Linus Torvalds rant about this. Why the fuck can't I use my dockstation that has 2 DisplayPort outputs and 1x HDMI to connect 3 external monitors? It literally works on Windows on the same fucking machine. On macOS all you get is mirroring.

So ultimately the reason is that Apple is completely oblivous to the millions of people who don't use monitors that are made or sold by Apple. Either oblivious or they flat out don't care.