[jwmerrill]

A lot of mathematical notation is optimized for hand-written algebra.

One of the main things you "do" with expressions like these is to transform them into equivalent expressions that have some desirable form, or examine special cases that allow simplifications, or examine the result of applying some additional operation.

Compact symbols and single letter variables reduce the amount of writing you need to do for each step of such transformations, but they also make the "structure" of expressions more visually obvious.

Programmers also spend time transforming programs into equivalent programs (aka refactoring), but refactoring isn't quite as important to programmers as algebra is to mathematicians and physicists, so (popular) programming languages don't optimize for it to quite the same degree.