You can load a bar, adjust a pin, or lean on momentum for almost every other lift in the gym. Not the pull-up. It's just you and your own bodyweight against gravity -- which is exactly why it's still the single most honest test of upper-body strength there is.
What's actually working
A standard overhand pull-up is primarily a lat, rear-delt and biceps movement, but EMG research on grip width shows the story is more nuanced than "wide grip for width, narrow for arms." Grip width and hand orientation change which fibers of the back and arms take the load, which is why rotating your grip over time builds a more complete back instead of a lopsided one.
Why most people can't do one
It's rarely about willpower. Most people who can't do a single pull-up simply haven't built enough relative strength -- meaning strength compared to their own bodyweight -- because they've never trained the movement in a regressed form long enough to earn it.
- Dead hangs -- build grip and shoulder tolerance for hanging under your own weight.
- Negatives -- jump or step to the top position, then lower yourself as slowly as you can control.
- Band-assisted or ring rows -- build the pulling pattern without needing full bodyweight strength yet.
How I use it in Nexus Discipline
I don't program pull-ups for ego numbers. I program them for quality -- full hang to chin over the bar, controlled down, every rep. Five clean reps beats fifteen half reps every single time, and it's the honest version that actually builds the back you're after.