It's probably corporate . To play Deep Rock Galactic with Xbox players on PC, you must buy a copy of DRG through the Microsoft store. Microsoft probably demands the same of ED and FD said nah.
This is wrong. It is easier to cross-save when you use a common sign-in and save method like Xbox but it isn't a requirement. Look at Halo. It's on Steam and the Microsoft Store. The signin method for all is signing into Xbox Live. Microsoft doesn't have steam players playing in a separate pool from Xbox players.
Frontier just needs to make a single sign-in source and save your saves on their side. Then you just link your accounts Steam, Xbox, Playstation, etc. to a singular frontier account and then when you sign-in on those platforms your save data is downloaded to one of those services and you play. All saves live on Frontier's side and these platforms are just requesting the most recent data.
With the size of team they have it is a little frustrating that they haven't implemented this especially given the teamwork aspect of certain parts of the game.