It's not about "ease", but about how the cars were assembled at the factory. For example, when we went through the GM-Arlington plant in the earlier '90s, we were standing there where the body came down the line, for the front fenders to be installed on a Chevy Caprice (body/frame car). As we stood there, a man came through the curtain, carrying a fully-assembled front fender assy out and placed it on the lh front section, the core support was loosely installed. He put the assy up to the body, put in 7 bolts, tightened them, and then returned to where he came from. All in about 2 minutes. I looked at my friend and said "We need to see that again." A minute or so later, here he came again, for another car. Instant replay. The common thought would be "Do you know how much time it took to take off that inner fender to replace it, with the fender still installed?"
This illustrates that sometimes, how things are done "in the field" differs greatly from how the cars are put together at the factory, in a particular build sequence. In the case of the fender assys, it's called "off-line assembly" where a complete assy is put together behind the curtain and then installed on the particular vehicle in mere seconds. Same with the instrument panels. Off-line assy and then (seats not installed yet) the workers walk the panel assy through the car, put in the attaching bolts, and their done. NO laying on their backs, as might happen "in the field". Parking brake assys are another off-line assy item.
Now, many of these things CAN be done when restoring a car "from the body shell up", as my associates did on a Plymouth Superbird. The instrument panel was done on a long work bench, where everything could be accessed easily. Then walked through the car and installed easily. The engine went in from the bottom, with the completed front frame section (which self-centered with their tapered-shank bolts). Of course, my friends had a large shop with a body-contact above ground lift. NO issues with scratching up the newly-painted underhood area, either.
But it all gets back to the build sequence at the factory, rather than what we might consider to be "conventional wisdom" out "in the field".
A side issue might be the cost of the paint that went on the '65-'68 cars, to put body paint on those inner fenders. Rather than the less expensive satin black that was "sprayed at" or "dipped into" for the inner fenders on the '69-'73 cars. Less paint spray = less VOCs into the atmosphere. The "dip" was a large container with paint poured on top of water, then the part was dipped into it, while on a holding fixture/hook, then went on down its way to air dry before it got to the assy line. Maybe even a bit ot electrostatic charge to get the paint and metal "together" a little better, as with the anti-corrosion treatment of the car bodies?
Enjoy!
CBODY67