You do have motorbikes with electric engines that are otherwise treated by law just the same as a petrol motorbike (riders need to have a motorbike licence, wear helmets, have insurance, the bike must have an MOT and be road-worthy etc).
Then you have electric bikes, which don't legally require any licence, insurance, safety gear. However they can't be very powerful - they can't go above 15.5 mph without pedaling and only have a 250W motor.
You'll also get electric bikes which have been modded to exceed those limits - e.g. a more powerful motor has been swapped in. That bike is then illegal and could be seized by the police.
These kinds of gangs often do use illegal bikes for their anti-social activities. It's just too difficult for the police to catch them.
Maybe some people did. Thing is there's a whole rest-of-the-world out there, and they didn't necessarily get the memo or are happy with the existing way.