@fetchacloth they do it because, as with the roundabout example I gave above, it's sometimes safer.
The idea of bikes being licensed and insured has been raised a bazillion times, it would be unworkable and create a huge amount of bureacracy and expense that would put people off cycling at a time when we need to encourage active transport.
Do you think the police want to be checking if bikes have insurance when they barely have enough time to investigate burglaries? Do you think little Mikey, when he uses an old bike from his uncle's shed, should have to complete official online forms and send off a fee then attach a licence plate?
Those things are merited for cars because of how heavy, fast and dangerous they are, and the risk of things going wrong if brakes fail etc.
With cycling, you'd be far better off funding things like cycling proficiency training and making bike lanes good enough for kids to ride on them rather than pavements. Carrot rather than stick.
When a bike hits a pedestrian, the cyclist is at a very high risk of injury as well as they're falling onto the road/kerb at speed, if that's not enough of a disincentive then I don't know what is.