Because you can't drive a car on public roads without mandatory training and passing an exam - so the assumption is that you know and understand the standing speed limits and obey them, but you ride a bike/ebike anywhere without any training or licences so the barrier to entry is lower .
Same way how(at least over here) anyone can ride a 50cc motorbike without any training whatsoever, but to ride something more powerful you need a proper motorcycle licence. I'd be happy with some sort of process where you can only ride a 250W ebike with no self-drive without a licence, but more powerful motors are available after passing a licence and with mandatory insurance.