Hey, thanks for your feedback!
We actually put full Steam Deck support in place on Steam before the game's release there. But without Steam itself and Steam Input, a lot of features were lost. Reimplementing and maintaining all those features outside of Steam, especially for a platform where it's not really welcome, is a new huge amount of work for no gain.
It's not about being upset or anything like that - it’s just a matter of practicality. Hope you understand, and thanks again!