Well, a total ban on 17yo's taking passengers would mean a 17yo couldn't repay their parents for years of driving them around.
Or if you restrict it to other young people, it'd mean a 17yo who is married can't drive their husband/wife. If they have a kid they can't drive them. Or if they help their parents care for their sibling, they can't drive them.
I think we'd end up adding a load of edge-cases which would make it rather complex. Though to be clear, I'm not saying we shouldn't bother, just that it's not as simple as "young folk shouldn't be allowed passengers"
Also, I wonder if in a few years, we'll end up with these "rules" driven by insurance companies rather than the government. Similar to the "black-box" insurance policies which are cheaper for many young people if they don't accelerate hard/brake hard/speed/etc, will we end up with some insurance companies offering cheaper insurance in return for a camera + microphone in the car feeding some AI to gauge the riskiness of any potentially-distracting behaviour?