Many (I think most) monitors have an audio output port. I think that's good enough for most people. Audiophiles use external DACs anyway.
You may still need an amp for electrically incompatible (high impedance) headphones.
But yeah, anyone "serious" would go discrete for all that stuff regardless. I guess this also lets Apple sidestep a bunch of fuss around non-stereo use-cases, for people who want quadraphonic or 5.1 at their workstation.