Well, that is not a reason.
Why can a software not support more buttons?
If there were more buttons on standard PS5/Switch/Xbox Consoles, I'm pretty sure, there would be software support for it.
If in a software they use x-input (the only method officially supported my Microsoft), then no, there is no way to use more buttons, the library doesn't have the functionality.
If they support ALSO d-input, then yes, they "could" implement other additional inputs, but those will work only for d-input devices (so this excludes almost the totality of xbox-like controllers that can't work that way).
I have the feeling that the
Xbox by default doesn't support d-input (but maybe could be hacked for that), anyway the developers make things for the majority of the users, and those are using "standard" libraries and "standard" xbox controllers.
I don't know what input system uses the
PS5, but I suppose D-input, because if I'm not wrong it should have a *nix OS, and also because the later gamepads have advanced functionalities like the touchpad, that is not supported by x-input as of now.
About the
Switch I'm not sure if it uses a different protocol from the previous 2... The 8BitDo gamepads have a different mode for the Switch, but I don't know the limits/functionalities of it.
So, if a game is developed for more consoles at the same time,
including the Xbox, maybe the controls are just standardized for the one that have the less functionalities (that right now is the Xbox)...