In the past many gamers built their own controllers because of good quality joysticks were scarce. Often today's game player builds his own equipment for economic reasons or for a more customized setup.
Joysticks are one simpliest pieces of computer equipment.
All they do is send signals of different frequencies to the computer. A simple joystick can be made from simple parts from Radio Shack. Once you understand the basics of joystick design, any kind of input device can be created, steering wheels, pedals, yokes, its all up to you and your imagination. Other input devices involve mapping keyboard commands. Use parts from old keyboards or buy a Epic board.