> kinda dumb when they have your plates they can just mail you a fine

thing is, in Germany and many other European countries there's a mandate to register your place of residence with the authorities in a timely manner (i.e. 2 weeks after moving in).

Americans and Brits don't have that, so "mail them a fine" is most likely going to result in the letter not arriving where it should.

I can't speak to the UK but in California there are various rules around updating vehicle registration when you move. Enforcement is pretty lax unless you drive something with exceptionally high registration fees.

There's strong wording about updating voter registration when you move, but I doubt there's much in the way of actual law. If there is it's basically never enforced as far as I can tell.