word choice - why do we say "she" referring to a general user of a program

I often hear people say "she" when they refer to a user of their application, for example. In documentation, or in email. Why do we say "she", why not "he or she" or "they"? I am not a native speaker, maybe this is a trivial question, but I do not know the answer.

Answer

It is something, in the U.S., that has developed as a result of "Political Correctness." Previously, the pronoun "he" was used nearly exclusively (barring the use of "they").