When Nikon introduced AF bodies (after the F3AF) the AF screwdrive mechanism was driven by an AF motor built into the camera body. In other words, it is a legacy system. The advantage of having an AF motor built into the camera body is it makes the camera body backwards compatible with every Nikon F mount AF lens ever made including third party offerings with the exception of two (the 80mm f/2.8 and the 200mm f/3.5 made for the F3AF back in 1983). As I wrote above, there are still several current lenses that have a screwdriver type AF drive and most of them are optically superb.
Keep in mind that many of us have much more invested in Glass than in camera bodies. Glass is a lifetime investment. I and many others continue to use old lenses, including: AF, AF-D, and even MF lenses 30 or more years old, along with the latest AF-S lenses.
The reason that AF motors were not included on the entry level bodies was to reduce the cost. Given that the vast majority of those that purchase entry level bodies will never go beyond the kit lens or lenses it made economic sense.
Since All AF lenses are chipped, the lens communicates with the camera body so it uses the appropriate interface to drive the AF be it electrical (AF-S) or mechanical (AF, AF-D). Good Luck and Enjoy your Nikons!