Most airlines flight plan on average standard weights for bags and pax. It would be too late once actual weights are in to run a flight plan and fuel up accordingly. If the TOW is lighter than planned the company gains on a lower fuel burn. If a pax turns up with overweight baggage they are charged extra. The average baggage weight is probably still the standard so the company gains again from the extra charges. It's the name of the game
This is why all airlines should weigh pax and there should be excess charges. Why should I pay for the fuel to transport the big fat blob next to me (not Mrs Exascot
) when I am well below the average weight?