score:11
Because governments of most of the world's countries have signed the 1949 Geneva Convention on Road Traffic or the 1968 Vienna Convention on Road Traffic. The conventions also unified most of the traffic laws, so tourists can drive anywhere without thinking too much about local regulations.
If you believe yourself to be incompetent to drive abroad, then don't. There are plenty of alternatives: public transport, taxis, Uber, car-sharing, walking, cycling, etc.
Upvote:6
Most written driving license tests are a joke, they ask you to regurgitate facts that you will never use again. What counts is the driving test, can you safely operate the car.
Most foreign governments look at your license as proof that you can safely operate the car, NOT that you know how far you have to park from a crosswalk. Driving safely is similar in most parts of the world, but it is up to you, the traveler, to make yourself aware of any peculiarities of the law where you plan to rent and drive a car.