Even if there are good reasons that some motherboard headers can’t all support the newest USB spec (due to bandwidth limitations), AFAIK there’s no good reason for the socket on the case not to support the newer standard. It’s just a few extra wires and a couple grams more plastic, and you can still daisy-chain an old-style connector for motherboards that need it.

Besides, usually the ports intended for the keyboard and mouse would be on the motherboard backplate itself. Front USB ports on the case tend to be used for things that do benefit from the speed, such as flash memory.