Why are there so many illegal number plates?

Why do the police not stop and prosecute cars that have "Doctored" their private number plate to spell their name? This used to be a traffic offence many years ago but is now totally ignored.

My neighbour is deliberately obscuring their numberplate to avoid ULEZ - should I report this?

We have a resident in our road who has 'stuck' leaves over her number plate to avoid detection from the ULEZ cameras. Her son is a serving police officer so we are reluctant to email/phone our local station...