Spotify pays content creators based on how many ads are played.
A white/brown/pink/etc. noise track/podcast is likely to be hours long, so that listeners can have it play long enough to fall asleep. Once asleep, the listener is still technically "listening" to the track/podcast, including the ads that Spotify then must pay the content creator for. With tens or hundreds of millions of listeners each listening to hours of content, this cost adds up.
Spotify has its own white/brown/pink/etc. noise tracks that it doesn't need to pay anyone for. Those are what Spotify would like to see its users stream, because it could save them - by their own calculation - $38 million.
Beyond that, the giant media corporations whining about the fact that money is going somewhere other than their own accounts is... par for the course. Really, this entire situation speaks to the absurdity of the industry itself and its own greed.