Whoever said pull-ups are easy is a jerk.
Once, when I was a kid, some rando on the playground told me that setting a record for pull-ups is the easiest thing in the world, and anyone can do it. I couldn’t do it, and I have a sneaking suspicion he couldn’t either. The pull-up is one of the staple exercises, the absolute bedrock of fitness. It is, in essence, a very simple exercise, to the point that yes, just about anyone can do at least a few pull-ups. But a big factor in doing a lot of pull-ups at once is how you maintain your form.
What does it mean to do a pull up? You grab a horizontal bar, let your legs hang down, and pull your body up until your chin touches it. Simple enough on paper, but what are you actually doing? Are you arms at equal length, or is one pulling harder than the other? Are you putting your whole body into it, or just pulling with your arms? Are you holding still, or are you flailing your legs? Little quirks like this can make doing pull-ups much more difficult, and you may not even realize you’re doing them. Try to take an objective look at how you’re pulling yourself the next time you try to do a pull-up set; you may find you’ve been doing something unconsciously that’s making the process more complicated than it needs to be.
If you find that vanilla pull-ups just don’t work for you for whatever reason, try a different kind of pull-up. For example, some folks find it easier to grip the bar with their hands facing in, while others prefer having them facing out. Maybe you could increase or decrease the number of fingers you’re grasping the bar with. Maybe the bar you’re using is just too high. It’s okay to dial down the intensity of the exercise as long as you make up for it with reps.