Because it would be difficult; it would amount to making my own camera from scratch. Right now I'm using a 35mm camera body. The lens mount and shutter come in handy. Although your idea is not a bad one, but I would still need a precise servomotor to drive the capstan, and would then have to make a camera and a film gate and a capstan. But I have shown that the approach I'm using can also work, and I only need a servomotor.