[scott_s]

For the same reason that if you put a hunk of chocolate in the oven and called it "hot chocolate," people would be confused that it's not a warm beverage of chocolate and milk.

That is, the phrase "virtual machine" is usually assumed to be the name for a piece of software that pretends to be some particular hardware. It is less commonly used to mean a "virtual machine", that is, not a noun unto itself, but the adjective virtual followed by the noun machine.