Consider this classic, very well written paper: https://math.berkeley.edu/~nadler/atiyah.classification.pdf
The “Generalities” section has only one page and hardly any equations, author simply defines notation he uses. And yet, it requires something like 2+ years of prior education in advanced mathematics (either in strong undergrad program, or in a graduate program) to really grasp it. Author even defines his notation: “E [is] the sheaf of germs of regular sections of E, and \Gamma(E) [is] the vector space of global regular sections of E, thus \Gamma(E) = H^0(X, E).”. Sure, he doesn’t define what H^0(X,E) is, but if I told you that it’s the degree zero sheaf cohomology group of E, would that make any difference to you?
The difference is that it is much easier to websearch a concept by name than by notation. "degree zero sheaf cohomology group" is a big handle to search, discover, and learn things with, whereas "H^0(X,E)" is a deadend.
I find that this isn't true. I spent quite a few years working on number theory problems on my own, just out of interest. I developed several techniques that I just couldn't understand when other people used them, because they had really weird notations, like |x-Σbⁿdₙ|<ϵ, with no definitions for any of the letters. (Feel free to guess what that means. Hint: nothing to do with set membership.)
Sure, you can work it out if you can compare several different papers, so it's only a barrier for a few days… but it's still a barrier.