Just to make it easier to think about, let's use an example. You mentioned cladding, so run with that.
Right at the outset, the cladding installation can't outpace the erecting of the structure. So you give the structure a headstart so that you have a buffer in case they hit a snag (if you catch the structure you either pay your crew to sit around twiddling their thumbs while the structure gets back underway, or you lay your crew off and risk that they're going to take a different job elsewhere).
The next problem is that even if you wanted to speed up installation, more hands won't necessarily do it. A solid installation crew can put up 40 frames a day. But paying someone to stand there isn't going to make the chain on the hoist move any faster. I guess it helps a little if you have a couple apprentices to break up crates and move material out of the way and take down safety fences and all of that, but the installation crew has enough latent downtime in their day that they can mostly do those tasks while waiting for other things to happen (while the guys up top are moving their rigging the guys below can prepare frames, and so on).
If you want to get really aggressive you can try running multiple install crews, but because of the way the frames lock together you'd have to co-ordinate that dance so tightly that it's not really worth it. And again, you can't get ahead of the structure, so why bother?
From my experience, most of the time that you're flooding a jobsite with labour it's more of a dog-and-pony show. The job is already way behind, so the subs throw bodies everywhere so they can shrug and say "Not our fault" when they miss the delivery dates. Or, if the job has gotten really nasty, the sub brings on a pile of bodies for any extra work that the GC orders (the subs will pay a worker $50 an hour to look busy, and then bill the GC $75 an hour on the timesheet)
But to cladding manpower, in the 80s a two-man crew (one on the top, one on the bottom) would put in 40 frames a day. Then, because the frames got heavier and the rules around lifting changed it was changed to a three-man crew (one up top, two on the bottom). Then the rules around rigging changed, and the new rigging required a second guy up top, so now you're up to a four-man crew to get the same 40 frames. And then more recently the rigging requirements shifted again and now you need a dedicated crane operator in addition to your four installers. The job hasn't changed, but it takes more and more bodies to perform it because the rules have changed.
