The simplest and most likely reason is just because they don’t have to. Playstation is leading Xbox by a lot this generation (so far, things can change) and PlayStation just has no incentive to add value to something they’re already selling record numbers of. Xbox is trying to attract customers, customer playstation has left on the cutting room floor, and backwards compatibility is a way to do that.
You can reasonably use Tidal to find new suggestions and plex to actually listen to stuff. That’s basically what I do. Tidal’s algorithm for discovering new music is better than Spotify’s imo, was pleasantly surprised when I switched because of plex/tidal integration. The biggest pain will be the initial Tidal setup with music preferences (likes/dislikes etc)
I will say though that Tidal/Plex integration is far from perfect, I initially got it to mix my downloads with a streaming service and quickly discovered plex cannot download Tidal songs which put a huge dent in my plans.
Plexamp is pretty great. Plex was functional when it came to music but plexamp’s features and ui really makes it feel like my own spotify