Why does starter only engage part of the time?

Why does starter only engage part of the time?
The starter in your vehicle works at a very low speed. It is only used to crank the engine enough to help it run from a standstill. The car's normal idle speed is +/- 700 - 800rpm. Even at this low speed it would be too fast for the starter to handle. Furthermore, keeping the starter engaged would cause unnecessary wear on the starter parts, as well as the flywheel gear (ringgear). It would also be another mechanical device the engine has to turn, thus decreasing the efficiency.