Because they probably didn't need to strip any old paint off first.
I my experience of painting houses etc (limited I know), the actual painting bit is the final 25-33% of the time. The rest of it is making the rest of it ready to be painted. So stripping off, preparing the surfaces, making good any bad bits etc ... only then can you start to paint it.
When they had fresh new metal to work with they could probably just paint straight away with zero prep. It may have even arrived at the site already primed. Plus it was probably easier logistics too in terms of seeing which bits were painted or not - if you are painting over the same colour it is hard to know what you have done and what you have not unless you are quite meticulous. However if you have a good contrast you could just give paintbrushes to 100 people and say "keep painting until all the grey bits are red" or whatever and not worry about the planning or tracking of which bits have been done when - just keep going until everything is red or whatever.
As a housepainter, this is probably the best reasoning and where my mind immediately jumped to. The prep work in painting is as you said the majority of the work -- many many many hours spent first slowly destroying something further, then bringing a surface back to the state where, finally, you get to make it all one solid color of protection, once again.
Generally the largest difference in a homeowner painted themselves vs a professional painter -- the amount of prep work required can be astronomical and absolutely mind numbing but without it, one will never end up with the final very nice and long(er) lasting result.
Seriously though, if you use house drywall painting over existing paint as an example, the actual "paint on surface" step really is short. In my most recent paint adventure the procedure was:
o move furniture
o lay down canvas
o tape
o sand
o [prime, depends on base and final color]
o paint
o wait to dry
o sand
o paint
o undo the coverings.
o enjoy.
It is very difficult to get all that done in one day, but I see new house interiors get basically painted in one-two day by a crew of like 6 guys, followed by an unending series of touch-ups where the painters did a half-assed job on trim, ceilings, and large featureless wall sections where it's difficult to see where paint is lacking.
IME Painting is boring and very easy to get wrong so I have respect for painters that patiently do the job right the first time.
Painting a room is one of those weird jobs that takes a lot of concentration (especially cutting in), and very little thought. I find it goes better with a beer in one hand. Given that basically every paint can opener on the planet is also a bottle opener, I suspect that I'm not the only one.
