[Zak]

an unmaintained and/or uninsured car is a danger to others

In most jurisdictions, it is illegal to drive a car without a specific level of insurance or with safety-related maintenance issues independent of whether it is being used for some commercial purpose.

[TeMPOraL]

Commercially used cars have different insurance and maintenance standards; this was one of many issues with Uber - drivers lacking proper insurance.

[vertex-four]

Most personal insurances will not pay out if they discover that you were driving for commercial purposes, as there is a different risk profile for commercial drivers.