No reason YouTube Premium should be more expensive than Spotify or Netflix.
As an additional note - I have previously subscribed to YouTube premium and constantly had issues with it repeatedly logging me out, and reverting back to showing me ads. Frustrating! Sure it wasn't intentional but felt like double-dipping.
Although my assumption (which could be incorrect) is that storage is not the main driver for cost. For Netflix and Spotify I assume their major cost would be licencing, which YouTube mostly gets to forgo. For YouTube, I would assume bandwidth / data transfer is the biggest factor.
Though in the case of Jay Foremans channel; those can actually be entertaining.
However, they've removed that plan again.
Come to think of it, I think I have 2 ATM; Apple Music through my Apple One thing, and through my ISP.