[DasIch]

Taxis operate under very different conditions than normal cars.

Most peoples cars are parked at least 20h a day. Taxis on the other hand are often operated in shifts and are in operation 24/7. This has significant effects on how much maintenance is required, how likely an accident is etc. this in turn results in potentially higher costs for insurance, requires more frequent checkups.

In addition a driver's license in Germany never expires, unless you violate any laws you can never lose it. That may or may not be reasonable for people driving a private car, it's a lot less reasonable for people operating a car commercially and potentially with passengers. Passengers who have the reasonable expectation of having a driver who's healthy enough to drive.