That is it Heathrow Harry - the carriers know they have created their own problem and enforcement is always done lightly. Whilst it is the case that the LCCs enforce the regs, they do not do so all the time on every flight. The publicity would lose them customers. So they strike a balance of checking closely once in a while and a few people get fined. But they pay up as they usually get away with it.
Since it is impossible for all carriers to agree to enforce all their baggage regulations all the time - what we have now will persist for some time.