Other platforms would be missing but that's not important. You would have no interest into showing all steam trainers (that's probably around 98% of our database). Epic games mostly work with steam trainer, same for GOG and Origin.
Windows store games are different. They often require a new trainer. Even if there is one trainer that supports steam and windows store it doesn't mean we don't have different code in it for both but just merged it into one trainer for convenience.
Additionally game pass is the first real "Netflix for games" service as you get games from completely different developers / publishers including AAA games. Filter trainers for this seems a good request. For other platforms you rather want to search for the game instead.
Thank you for clarifying 0x90 🙂