AFAIK game pass is losing money at the moment and is still in the "pay a premium to get content on the platform" phase. They haven't switched to the Spotify "screw the content providers" phase yet.

And they may never get there because Microsoft can float loss leaders indefinitely if they can justify them in strategic terms.

Spotify was in that phase since it's creation. Contrast paying amounts between Apple Music and Spotify to see that in numbers.

Game Pass has the huge advantage of being about games, it can retain a much lower game library number (~100 titles) but still keep people subscribed by rotating the games around first party titles. This means that until the landscape changes (Nvidia's attempt being squandered nullifies a lot of potential here of a landscape changing), being an invited third party means good income, due to the revenue being split ~100 ways rather than ~11 million ways.

Of course the above is a simplification, as each game gets its own separate contract agreement and the payout isn't based on how many times a game is played.