> subtract a percentage for themselves
Streaming platforms (let's not just pick on Spotify, though they also pay worse than others) just do this arbitrarily. They decide how much to skim off for themselves before graciously paying artists. This isn't how it works in basically any other industry where one group of skilled individuals create content that is then sold on some marketplace - instead, the sale price and proceeds are visible to all, and the middleman takes a well-defined, reasonable cut of that.
There's no need to compare streaming platforms to other models of paying musicians, in order to know they're getting a bum deal. This model is not fair in the slightest, and it's perfectly alright to be upset about it.