It's not hard — For games that have been designed with consoles in mind, APIs are available to easily port gamepad controls over to the PC version. Valve recently added support for the PS5 DualSense to its Steam Input API, for instance, and Microsoft has a similar API for porting Xbox games to desktop. There's really no good reason to not have gamepad support, and yet some still don't.
"If the number of customers with controllers is really high, but not many of them are using controllers in your game, it might suggest that you haven’t done much (or any) work to support controllers in your game," Valve says.