I noticed refueling automatically updates fuel weight
Yes, but only on airplanes flagged to use the default fuel page.
why not have GSX update appropriate payload weight when passengers/cargo board/deboard? Would add even more realism to an already very good product.
It does that already, but it requires airplane support for it, like in the FS Labs Airbus.
The reason it cannot be done automatically like the Fuel, is that, while Fuel Tanks are clearly defined in their meaning, payload stations don't have a "type" so, a payload station can be anything, from a single person to a whole cargo section or a passenger section and there's no way to know which one is which.
So, there's a different system in this, case, which tells the airplane when passengers are entering/exiting, so the airplane code can adjust its own payload, knowing what its stations represents.
