Mathematicians don't just use greek symbols, they use upper-case and lower-case, they use different fonts (script, blackboard bold), they annotate letters with symbols (^, ', etc), and they use bold or italic.

As to why mathematicians use so many different symbols, the first paragraph here gives some idea (http://www.w3.org/TR/MathML2/chapter6.html)

There have been many new signs developed for use in mathematical notation, and mathematicians have not held back from making use of many symbols originally introduced elsewhere.