As someone trained in mathematics, I can tell you that using single character variables allows one to focus better on the concepts abstractly which is one of the goals of mathematics. That is to say, it is a practice well-suited to mathematics.

It doesn't carry over to programming where explicit variables are better suited. In mathematics one is dealing with relatively few concepts compared to a typical program so assigning a single letter (applied consistently) to each is not a problem. This is not so in programming, except for a few cases like using i and j for loop variables (back when programs had explicit loops).