To add: the top video only has 10k views over 2 months. YT is watched by billions of people every day, 5k people a month accidentally going to /playlists probably isn't worth the cost of kicking back their username and paying the engineers to make the commit reserving that url.
Honestly I'm surprised that YT would let users have anything like a top level path; I'd expect it to be all youtube.com/u/myuser/$FOO where FOO can be a video or playlist or whatever.
I have to imagine it's for backwards compatibility. Seems to have been a choice made in 2008[0] if not before, although new handles are primarily accessible via `/@handle`.
Typical people don't add stuff to a URL, but if they see a link that says "youtube.com/playlists", they will probably not expect it to be some garbage channel not affiliated with Youtube.
