I may be way off base on this, and if so, the developers can smack me in the head.
Mouse operations are the domain of the OS interrupt system, and QB64 has to play nice and not supplant that system, or bad coding could seriously crash it. _MOUSEINPUT is QB64's way of going to the OS, with cap clutched to its breast and saying, "Please sir, may I have some more..."
Whereupon, the OS is notified, and it can determine if the resources are available before granting the request.