For textbooks, it's a non-concern because the meaning of each equation is explained in the surrounding text. The amount of new notation (by which I mean symbols such as / for group division or notation for differentiation and integration, rather than variable names) introduced by any given textbook is not that much, and as others have pointed out, by the time you finish reading the associated theorems, examples, and have done some of the exercises (very much underrated part of learning mathematics), if you understand the concept represented by the notation, the notation itself is trivial to remember.

Variables, as opposed to notation, tend to be local in scope, so 1) it isn't vital to have a memorable name and 2) it wouldn't really help to have a 'proper' name, either. As an example, if we're talking about two elements from a group, we usually talk about two elements, a and b, in a group, G. Could you call them elements 1 and 2 from an ExampleGroup? Maybe, but there's no clarity provided since these variables are only scoped for a given section. Additionally, longer equations means that it takes longer to read through and check steps like algebraic manipulation in a proof, or application of a lemma or theorem.

What is globally scoped are things like theorems, lemmas, definitions, etc., and those are easy to track down since they are by convention numbered after chapter and section.