It's not why. That's just a talking point that gets repeated all too much. In most places you don't need a license to operate a boat and they go as fast as you can afford.

Despite what the other poster is saying, there's no law of physics that says speed can only be unlocked by licensing.

I dunno about "most places". I definitely had to get a boating license, though for some reason the law had an exception for people born before 1978. A cursory search show most states have some sort of legislation regarding it. Many other countries have different legislation based on engine horsepower and age, but it's definitely regulated to some degree most places I've seen. I don't suppose you'd be able to paddle hard enough to cause serious damage

Oregon resolves that by regulating -on motor" only speeds to something a little under 30 and less than a kilowatt.

Everything else is simply a pedal assisted ride.

And a vehicle needs to offer pedal power to qualify for unlicensed use as an e-bike. Otherwise, it becomes a motorcycle and requires registration,

I used to as a kid myself. The norms were generally slower overall, limited by muscle fatigue mostly.