Borrowing an instructors car for test

This is how the fone call usually goes:

"Hello, my son's got a test booked for tuesday and I was wondering if he could use your car?"

"Well I don't usually just let people use my car, I'd like to know if he can drive first."

"Well he's been with an instructor for months, but he won't let him use his car."

"Why not?

"He says he's not ready, but he's had 20 lessons."

"If he's not ready, then he needs more lessons."

"But he's a good driver, he drives my car all the time."

"He could do the test in your car then."

"But what if he crashes it?"

"If he stands a chance of crashing a car, then he's not ready for his test. And if you have that little confidence in him, why should I let him loose in the most important tool I have, knowing that if he crashes it, I am out of a job until it is fixed?"

"Oh, right. So you won't take him?"

"No."

Why do people do this? Why do people think that they can fail a test and not take any more lessons before taking another? Why do people think that proving that you are capable of the MINIMUM SAFE STANDARD OF DRIVING is something you can just have a go at?

Is there any chance we can blindfold these parents then ask them to sit in the car with their kids who are so good at driving for a week before they are allowed their licence? And if their parents don't agree to this, could we tell them they need more lessons?

"But he's had 20 already......."