Nobody is checking. They're in private cars and small vans, so they aren't on a tachograph. They're technically self-employed subcontractors, so Amazon has no statutory duty to monitor driving hours. IIRC they're paid per drop rather than per hour. I have no doubt that a significant proportion of couriers are habitually working unsafe hours. In my neck of the woods at least, couriers working for Amazon or Hermes are overwhelmingly a) Eastern Europeans who look chronically sleep deprived or b) semi-retired Brits doing it for a bit of extra cash.