I think the difference is that offline playback and background playback on iOS/Android can be unlocked through YouTube Premium so those apps directly interfere with YouTube's bottom line. YouTube-dl I don't really see as directly competing with that because it's not trivial to download a YouTube video from it to your phone.

And given how unlikely people are in the wider non-technical audience to god-forbid, run a command line program, I guess they really just don't care.

They do take easily accessible apps that use youtube-dl under the hood pretty seriously. I guess it depends on how much of an effort it is for them vs how much of their bottom line ytdl is cutting into.