It’s not. It’s 100% a MacOS limitation, because you can use MST on Windows to drive multiple displays on Mac hardware, the same is likely possible with Asahi Linux if they want to support it. Apple doesn’t want to use MST because they want you to buy into their Thunderbolt displays for extra monitors, or you can just use DisplayLink if you’re desperate like me.
It’s an arbitrary one. Most Windows and even Linux laptops can display to multiple monitors without extensive display controllers. For example, my two HDMI USB-C hub can be driven by a Windows or Linux laptop and independently display two monitors, MacOS mirrors them. On my M2 Max MBP if I connect one USB-C hub to one monitor and another USB-C hub in another USB-C port to another monitor, they work. No DisplayLink.
Yeah that thing is artificial limitation. Asahi folks might get MST support working, but I'm not familiar with the chip so it might not be. I was thinking about total display limit.
Also Thunderbolt dock can be had for not a lot if you search enough. I got mine at a reasonable price.
I use a DisplayLink dock at home because it works, it’s just kinda annoying that I have to hack multi-display on an expensive laptop. When I go to the office I just deal with double-docking, it’s still a better display situation than when I was running Ubuntu and GNOME.