So why is that a harder to screen for that potential problem than any of the other ones? I can see why it's a concern, but it's also a concern that they are lying/deluded about their actual skills or are going to show up to work hungover and useless or slack off too much.

If you said those other ones were a legitimate concern for avoiding hiring young people I would say "... I guess, so learn to interview properly and lower your risk of that". The scale of that risk doesn't fit the scale of the issue at all.