Apple's display controllers are quite sophisticated and large - a single one is
larger than
two performance cores on these chips (almost half of it is cache memory, to keep power usage down while the screen isn't changing). The M1 Max has more silicon for display controllers than CPUs. It is not a trivial amount of silicon area. The baseline chips can't physically fit more than one external display controller without making them significantly larger and more expensive.
DisplayLink of course works, but it's an ugly virtual screen thing using compressed data over USB3, not a true directly connected external display.