Why is the fax an output device?

Answer 1

Computer systems are composed of three parts - Input, Processing, and Output. You put stuff INTO a computer with the parts of the input system - keyboard, ports, etc etc. The processor process the data. OUTPUT devices take information from "inside" the computer, and convert it to an external form - e.g., the paper in a printer. Plus, most printers don't have any functionality like a keyboard with which to communicate with your processor (notwithstanding the settings you might be able to manipulate with some printers.)
The printer is a standard output device since the data goes out as opposed to a keyboard or mouse whose movement signals input to the computing device. Some things can be both such as a touch screen display.

Answer 2

The fax is an output device because it receives electrical impulses and prints an image that can be read by humans as its OUTPUT

The scanning part of a fax is an INPUT device because you put a picture (or typed text) into it and it sends electrical impulses somwhere (to another fax)