Answer: Because the old school Kodak film has a width of 35mm. So with 24x36mm frame size and perforation, the punched holes up on the top and bottom of the film, it has the width of 35mm. That is why it has been called a 35mm film ever since. And since that has been the standard film dimension for ages, photographers refer that as the "full" frame film. Numerous professional photographer actually refrain from using that term because it doesn't give you any idea of the measurements.
