Driving test question: Why is this the answer?

That is the way the law is written. At uncontrolled (no stop or yield signs) it the person on the right who has right of way.

Because if the only other cars at the intersection that arrived at the stop sign before you were on your left or in the oncoming direction across from you they still have the right of way over you. If two vehicle come to a stop at a 4 way stop at the same time the car to the immediate right has the right of way.

It is physically impossible for two vehicles to stop at the exact time.

THE DRIVER TO THE RIGHT HAS RIGHT-OF-WAY ONLY IF THE VEHICLES ARRIVE SIMULTANEOUSLY. OTHERWISE IT IS THE FIRST PERSON AT THE INTERSECTION THAT HAS RIGHT OF WAY OVER THE OTHERS. 

If the cars arrive at the same time, then the car on the right has the right of way. So you yield to the one on your right.

If the cars do not arrive at the same time. then the first to arrive has the right of way. Yield to them.

If one car reaches the intersection first, he can go.  If two cars get there at the same time, then it's the one on the right.  You must yield to the driver who arrived before you.

Because that is what was decided as the best way to handle a 4-way intersection. Makes sense to me. Or would you prefer that the next time you get in line at a store, the person behind you goes to the cashier before you?

cause that would be the answer to a different question...  when two vehicles simultaneously arrive at the intersection of a 4-way stop sign, etc

The first driver at a 4 way stop has the right of way.