Check out this modern cycling video. The narration is drowned out by the music! Awful!
I say this with all respect, it's probably because it's not worth the effort.
Sure, Youtube could create a tool where video publishers add music within the website instead of editing it in before upload. It wouldn't be around for very long and would break any videos that used the tool, going on previous experience with Youtube.
edit - I've just checked, and it is technically possible to add music to YT videos within the YT Studio. I have no idea what effect this has over and above baking the music into the video.
THEN Park Tools could re-edit a version without music to all their videos, re-upload a music-free version, re-add the music in Youtube, and bingo! But what's in it for them? Why do all that extra work?