The speed limitation is down to the fact that they don't need insuring. Like pedal bikes the idea is that they will cause limited injury or damage to property in the event of a crash. Up things to 25mph, plus the frame/battery weight needed to add strength and safety to accelerate and brake swiftly, and suddenly you have plenty of energy to damage a car or other piece of property or hurt a human (yes I know cyclists also occasionally hurt humans).
So therefore they will need insurance, and therefore need registration. And guess what, you can ride an electric scooter or bike at the speeds you mentioned, you just need to register and insure it. It is entirely possible and people do.
What you are suggesting is a way to ride fast vehicles without the social contract of insurance and traceability.