The smaller the a/c, the more critical weight is. Those that operate craft such as the 1900d know that. If they ignore their responsibilities, then they will be held to account. For large jet transport, the same is true but tolerances are greater and they have more power to compensate.
Over the last 50 years, average weights for pax have worked very well. These averages are updated every couple of years. Baggage weights are, as I understand it, now automatically fed in due to the computerisation of the weighing system.
If every pax was weighed, the cost would go up and most pax would ask why the need to charge more money to weigh pax when average weights are well known.