Why can't you trade in Rocket league?

Why is not that one can, for example, as an xbox player with a PC player swap but can play together

Can't tell you now with certainty but I think that it has something to do with the money so the pc player just buy their keys on the PC and ps4 players on the PS. I'm assuming that when you buy keys, you also pay a certain amount to the platform

Because each platform has different market prices for the items, and above all because there are platform-specific items.