>Why can't kernel itself ship a more advanced terminal?

Probably because nobody wants to put opentype font rendering and input method support and various other things from a window system into the kernel? This stuff all has to be in userspace so you're better off just running a minimal X or Wayland server at that point.

Wider color support hardly requires having complex font rendering features.

Because noone is paid to do it.

That's obviously not the reason, there is a lot of good stuff done by volunteers, it's niche and not enough ppl care about it.