Lee Jay wrote:
I've wondered why the payload isn't installed in the payload bay either
at the OPF or the VAB. Why is it often (always?) done at the pad?

When you consider how long it takes between the shuttle bay doors closed at
OPF and the actual launch, your cargo wouldn't be able to carry any
perishable experiments.

If the whole flow of work had been done so that they could be like a Soyuz:
move to the pad and launch the next day, then perhaps they could have done
payload loading in the VAB.

The one advantage of the current process is that it frees the VAB quickly.
This way, they can pipeline the work with an orbiter in OPF, one in VAB and
one at the pad, and thus theoretically have more frequent launches.