In case of math literature, I’d say it is mainly because the equations would be horribly long. Things like integral computation comes with lots of details, or some kind of long exact sequence. Experts would have trouble understanding it, while normal people almost certainly have no interest in understanding the formulae.

Also, often there is no meaning to the variable. Like, in a real function f(x), x is just “a real variable”. There is no additional meaning to it.

As for other fields, I dunno. They might have copied math literature style or something.