The reason is that it makes everything much more compact, which facilitates symbolic manipulation by reducing the space that symbols take, so that the differences between each step can be easily spotted, and in particular so that it’s easier to write down each step, which is especially important when working with pencil and paper.
The problem with short cryptic symbols comes when there are so many symbols that it’s hard to keep them straight, when symbol uses fall far from their definitions, when the same symbol is overloaded to mean different things, or when the author’s and the reader’s cultural conventions about symbols are different.
In a programming context, short variable names are extremely handy when either (a) the scope of the variable is limited, or (b) there’s a strong convention associating the name with the meaning, so that someone seeing the name knows immediately what it represents. For example, writing pi instead of circle_constant, writing i instead of loop_index, etc. tends to make code more readable rather than less.