That way you try to bring the quality of drivers up to that of the instructor, not a cargo cult of family members who might be horrid at driving.
Sure, but "experience" doesn't have to mean you're out there on the highway with everyone else. You don't start scuba diving in the actual ocean. You get experience in a pool for a while first.
This is a common misunderstanding of what is really needed in driver education to improve the average driver. The average American driver really lacks a lot of skills and a clear understanding of optimal driving behavior. Driving on public roads should be optimized for safety, but most drive as if it ought to be optimized for speed.
It would be a good improvement if all drivers were required to take a Smith system or similar defensive driving course annually in order to retain their license.
But somehow driving, and the daily experience of it is: fuck you, me first. And it shows. It's an adversarial system. I'm really impressed that it works as well as it does, despite being the #1 killer of teenagers, and 40,000 total deaths on the road per year.
So you end up with the current system which is fairly similar across states and countries with somewhat greater or lesser rigor depending on the particular place.
Driver's ed at least used to be pretty common,in part because it gave a discount for the (expensive) insurance covering younger drivers. I don't see anything special about the typical driver's ed instructor anyway. It's not like the typical driver's ed is some expensive defensive driving course taught by an ex race car driver.
Furthermore, the typical system is that someone gets a learners permit first which lets them drive only under a limited set of conditions so they gain experience that way.
And, honestly, you gain experience by doing things not taking classes.