Because not everyone have same interest, and the player never can remember the track you selected, so if the default track not your favor then it's annoying since you need to re-select every time you want to watch it.
For example some of hentai anime I got have German dub which is worse as fuck and I want to hear the original Japanese voice so I must re-select the track every time I watch it.
Obviously, I can hardly study
every single media player out there, but as far as VLC and MPC-HC go: switching audio tracks is literally ONE key press away. Otherwise - configurable to work that way.
As for the anime example: both media players have a setting that tells the player to auto-load the corresponding audio/subtitle track if present: "preferred language" for VLC, "default track preference" for MPC-HC. Although, comparing "moaning" videos to "actual spoken language where understanding is critical rather than merely preferred", in the first place, sounds wrong (pun not intended). Vastly different cases, at least until "moans in an incomprehensible language" becomes a thing.
Genuine question: "Default track not your favor", where the other track is "Male VA" who can hardly be [ever] heard (if Saeko and Cammy are anything to go by) over the Female VA present in both versions, anyway? Really?