We use Greek characters simply because it's convenient to have additional symbols at hand. The choice of Greek is historic, but one also sees a little bit of Cyrillic and Hebrew. Notation has evolved since ancient Greece. Euclid, for example, is quite readable in translation, but he did not use modern shorthand. See for yourself: http://aleph0.clarku.edu/~djoyce/java/elements/elements.html
That assumes single characters are the only appropriate variable names. There's no reason maths couldn't use a * symbol as multiplication instead of assuming multiplication for any string of characters. People who enjoy the current state of mathematics will probably think the OP is trolling, but it's a reasonable question. Perhaps it could be better put as 'Why doesn't maths concern itself with readability like progr…
A typical program is written one, read a few times (perhaps years later) and executed many times.
A typical math calculation is typed once and read only a few times (a few minutes later) and discarded ("archived").
In the draft, I usually find myself replacing sin(x) and cos(x) by s and c, because it's only a little calculations and in the following 10 minutes I will remember what it means and there is no other s or c in sight and I get bored of writing all the letters.