From what I have heard this is because Spotify has many people on their free tier, versus profitable competitors like Apple music which give a trial but are paid-only. If I understand it correctly Spotify's ads on the free tier don't generate the $10/mo revenue they get from paid users.
Apple doesn't break out revenue for Music and it's highly unlikely they are profitable, or if they are, it ain't by much.
The labels have a monopoly on talent and content, It's impossible for anyone to make significant money in the streaming game when they have a stranglehold on the supply.
Artists should be mad at the labels THEY entered into contracts with, not spotify.