Why don’t modern operating systems include built‑in retro emulators?
Surely many users would appreciate nostalgia features—especially if the OS vendor handled all the legal and copyright clearances.
It's a legal minefield that offers no benefit for Microsoft, Apple, or Mr GNU/Linus. There are plenty of well loved, highly accurate emulators for various systems available from 3rd parties today.
I think you overestimate the number of people into retro computing. There is no business incentive and only downsides.