That's definitely missing some reasons. Back in the day onboard audio also was just terrible. Like it hissed at all volumes & other similar just bad dacs and bad amps and bad shielding problems. Which soundcards mostly all fixed.

These days basic "audio correctness" is readily available from onboard audio, though. Motherboards have gotten much, much better at noise isolating the audio area, and dacs & amps have generally improved.