With Sonos it’s just accessing the APIs for respective services, it does not have an internal database like Roon which is for streaming services and local music therefore it’s pretty easy to implement as the app is controlling which account is used at any one time to stream to either one or multiple.devices.
Roon has a local database that when you add music from it to your Roon library it becomes part of this. So Roon would need to have multiple databases running concurrently to manage different users accounts. Given how taxing one library is on the hardware at times managing multiple databases could be a nightmare. All stuff that manages to have multiple accounts does not do what Roon does with the streaming services its a very different challenge to achieve. I can guarantee you can’t have multiple accounts active on one device at any one time so what happens for Roon it would not be able to manage your account at all if someone else is using it in one room and your in another. Sonos doesn’t have a central server it’d point point so again easy to implement. It would be great to have family or multiple accounts but i imagine it’s not easy and likely API restrictions don’t allow how they would want it to work.