I don't think they're intellectually incapable as such, I have the impression that most drove responsibly alone. Pretty much all the really reckless driving I saw was showing off or egging each other on and nobody had the social maturity to stand up and be the uncool party pooper.
Probably not intellectually incapable, but I think that the many young drivers greatly overestimate their own abilities. Including when they're driving alone.
To use myself as an example: at 19 I had lightning reflexes and good technical driving abilities, but to think back at how I sometimes drove then (including driving alone) gives me the shudders nowadays (I'm 40). If something unexpected had happened I could have killed myself or someone else due to sheer inexperience. Maybe luckily for me, something unexpected happened while driving at only 50 km/h which caused me to not be able to brake to a stand-still in a moment of inattention, and I hit the car in front of me. Entirely my fault, no injuries to anyone, but substantial material damages. It made me realise how much one needs to expect the unexpected when driving in traffic.
So, nowadays, I'm a very careful driver. I believe that between my 20 years experience and my previously mentioned (but diminishing as the years go by) technical driving abilities I'm at a peak of my lifetime safe-driver-factor. Still, contrary to most men, I consider myself only an average driver. It goes downhill from here, and I'll drive accordingly. My insurance is very, very cheap, and in my case I think it matches the risk
And, I have onions on my belt, or something. Sorry for the rambling.