External AF motor for MF lenses

Why doesn't anyone make such a thing? It could involve these components:

• an adapter, the size of other adapters available for various third-party MF mounts, which would communicate with the camera for autofocus commands

• a motor which could rotate a clamp, which could be affixed to the MF lens's focus ring; the motor's external housing could be stabilized, perhaps by affixing it to the camera's tripod mount or the battery pack discussed below

• an external battery pack to power the motor, which could be a fairly small plate-like device designed to affix to the bottom of the camera, via the tripod mount, and could offer a pass-through mount for a tripod/monopod underneath, and also could help hold the motor's housing steady

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