I can't say I share your experience overall but it really differs on an application basis, some are more thread aware than others. Since I got my 3900x I regularly run multiple VMs, FLAC files, encode video, game, dabble with some compilation and stream without issue on both Windows and Linux. Generally speaking I don't see many idle cores when there is demand.
Playing: Ys VIII, Hades, Demons Souls
Watching: The Expanse, Tenet, Peninsula