for the same reason you need a license to drive: regulations.
That seems an a obvious requirement for the insurance, but Uber aren't complying.
Not everything needs to be seen with a negative (American?) view of government. Uber's competitors, including those with apps, have licenses.
It's an excellent system that warrants repetition in other industries.
Regulation done right greatly improves one's position soliciting services - I want to know if my driver has had recent accidents, speeding tickets, has poor health, doesn't pass a DBS check, can speak my language (very useful in cities you don't know), that their vehicle is safe.
Good news is, a driving license for with passengers is just an additional test that will overall set you back less than 100 Euros.
And, additionally, your insurance becomes void as soon as you have a paying passenger, or are driving to one. But cars on the road need insurance, and Uber’s insurance only is valid while a passenger is in the vehicle – not on the way to the passenger.
I even rented a car at the airport and Hertz clerk haven't mentioned anything, despite seeing me approaching with friends. They only asked if there are drivers, not passengers.
So how does it work?
Not passengers in general.
Your Hertz terms will clearly say that you're not allowed to use the car to carry paying passengers.
This allows up to 8 passengers. If you want more, you need a license for bus drivers, Führerschein Klasse D.
If the passengers pay you less than, or equal to the costs that you had during driving, you do not require any of this, and it is considered noncommercial (for example, when carpooling and passengers pay for gas)