Historical and software architecture. Roon was first only for local libraries. Streaming had to be integrated later, and it wasn’t / couldn’t be rewritten from the ground up for that.
There are many consequences of this that are limiting and Roon knows that at some point they will have to solve this, but it’s a lot of work and not without frictions (requiring always-online in Roon 2.0 because of heavier reliance on cloud infrastructure wasn’t met with 100% approval, for instance)
Processing power. Adding something to the library also creates a selection of data that has to be queried, and even the very large libraries of some users with 1 million tracks are still vastly smaller than 90 million albums on Tidal or Qobuz. Running a query on 100K or a million in-library tracks is still within the possibilities of a local PC, running the same query on a billion tracks on the streaming service isn’t.
This is also why search had be moved into the cloud to be able to improve it to the level users expect. This post explains the necessary search infrastructure and probably also tells us something about challenges that would be posed by running a focus on all of Qobuz/Tidal: