I still don't know why any iPhone people have Spotify over Apple Music
Your universe must be really small then.
Let's say you live in any of the majority of European countries where Apple still hasn't rolled out HomePod. Those are likely to be the same countries where Google pretty much owns the smart speaker market (since Amazon are dragging their feet just like Apple). Conversely, in pretty much the same countries you can't yet use Apple Music on Google devices, only Spotify or YouTube Music.
So in Scandinavia for example, even as an Apple fanboy, you're likely to have a mix of Google and Apple in your home, otherwise you can only do a fraction of all the Smart Home stuff you need. Now Spotify fits right into that because it's so ubiquitous and works across all your devices: Macs, iPads, iPhones, Apple TV, Google Nest devices, Google Chrome devices, Samsung TVs etc etc etc etc. And you can fling those streams between them in all sorts of combinations. Tell your Nest Audio speakers to play music on Spotify, move it to your iPhone w/ AirPods, anything you want (except Apple Watch where Spotify is only a remote control). Switch from Spotify to Apple Music aaaaand... music playback is cut off from your smart home stuff and sent 5 years back in time: iPhone with headphones and not much else, except maybe the odd speaker w/ AirPlay (Sonos, Sony etc, not so much smart home but more like glorified bluetooth speakers). Add to this the fact that a lot of EU providers offer Spotify Premium at reduced price as part of their plans. And the fact that Spotify has podcasts and music in the same app, and that they now have exclusive deals with Rogan, Armchair Expert etc.
I live in Sweden and I use Apple Music _only_ because I need it synced independently to my cellular Apple Watch. Everything else about AM is just a very long list of disadvantages where I live. A comparison table where Apple's column has 95% red X's, basically.