Very often, when a song comes up, I will want to delete it from my current playlist because it no longer interests me. In many years it has been practically impossible in the UI. The only way to do it is to hunt through thousands of songs, then click on a context menu to delete it, despite this being something that would be incredibly easy to add to the current song interface and accomplish in one second (Adding the same song to a playlist is an option there).
So why is this the case?
I recently realized that preventing me from cleaning up my playlist means MORE SKIPS for Spotify. They can shove more ads in my face by forcing me to listen to more stuff I don't care about over time. It sounds silly but I would bet money that this is the reason considering how technically simple this feature is and how large the value would be.