Me, being a naive speculator: Maybe it has something to do with the time. The original contracts between spotify + labels were probably written 15 years ago. Over time they might have changed numbers like how big spotify's cut is, but never revised the rest of the blueprint contracts.
So, my bet is laziness / not caring enough.
Labels do prefer Tidal because it's a service made by the music industry. However, Spotify is lacking features it used to have because they are product driven and such integrations are not on product people's radar (like you said - lazy and don't care).
Yes, labels and artists get a bigger cut of the subscription cost from Tidal. And before the buy-out by Square last year, Tidal's parent company was majority-owned by Jay-Z and had lots of buy-in from music industry insiders.
This is somewhat tangential, but I feel like this happens often, as internal power and culture shifts away from being developer-driven to consumer- or manager-driven.
This doesn't happen for every company, thankfully.
And, to your point, not every company follows one. But most do.
It happened to Twitter didn't it?
It's since evolved into something else, but it's not surprising that Tidal can get some unique deals due to its close industry ties.
When Spotify was young they were extremely developer and ecosystem friendly. It gets progressively worse and worse over time.
The biggest change I personally suffered from is when they pulled out of their integration with Djay, a DJ app. This integration was amazing for bedroom DJs like myself, being able to use Spotify to organize DJ music and DJ directly from it. Then they sunset the entire integration.
Now Djay and even bigger apps like Pioneer Rekordbox integrate with Tidal... Do the labels prefer Tidal over Spotify for some reason? Or did Spotify decide to get out of this game for reasons of their own?