They already do. If I know a number is required, I don't have to try any passwords that don't have a number.

They invariably do. They always constrain the list of things that a fully random generator could possibly make. They never add to that list.

Even rules like "can't use the same character twice in a row" constrain the list at least a little. That one makes it harder for dumb people to do dumb things, but also makes it harder for smart people to do smart things.