I get that it's defined that way, but I'd really like to know why.

I can see the value in saying that struct x* isn't compatible with struct y*, because they could have different alignment or packing rules. But struct x* and void*, which is already special-cased to allow assignment without a cast? Why aren't these considered compatible in function pointer parameter definitions?

Is there any work involved in casting void* to struct* (on any architecture) that a plain function pointer would miss out?