In short, because there is no room to do that, without prohibitive cost in additional optical elements.

The lens and body are designed to provide the correct distance between the optical elements and the sensor. On a Nikon, that distance is on the order of 45mm (from memory, it can be looked up somewhere or measured on cameras with a reference mark on the body).

Inserting anything between the lens and the flange will increase that distance, and the lens will no longer be able to focus all the way to infinity.

While this can be a desirable effect for some situations (google for extension rings and follow links to learn about macro photography techniques), it makes using the lens for general shooting nearly impossible.

Note that in order to reach the screw drive head even if the motor were mounted entirely outside of the lens body, you'd still need to have several mm of working room. The practical extension ring I keep in my bag for use behind my 50mm f/1.8D is 12mm, and at that thickness the furthest focus available is at under 6 inches. 12mm isn't a whole lot of room for mechanics....

In principle, you can fix the problem of thickness with relay optics that compensate for the extra flange thickness. But each new piece of glass you put in the light path causes loss, potentially causes flare, decreases image quality, and costs money both to design and manufacture.

Check out the price on 1.7x and 2x teleconverters, which are optical systems similar to what you'd need to use, but which are solely used to extend the effective focal length of longer lenses. They not only are expensive, but the 2x converter will also steal more than a full stop of light, making autofocus harder to execute at all.