Question about passenger boarding

[jboweruk]

I've watched several programmes on TV about airlines and they don't seem to delay for a passenger who's late, you get told when you get
your ticket that you have to be at the airport AT LEAST 1 hour before boarding or you get left behind, simple as that. On the Airline
programmes I've seen them leave ppl behind just for being 5 minutes late. This is because the pilot has to get his manifest before he
programmes the FMC, he absolutely must know what weight he is carrying in good time. I'm sure any real Airline pilot may be able to
explain better why this is, but I'm confident it's to do with weight and balance issues.