I just don't appreciate being lumped in with all the chavs and idiots that frequent the roads where I live.
I don't drive everywhere bouncing off the limiter, showing off my crappy Nova with an exhaust, or crashing into anything that dares get in my way.
It's not just that though, it's also stuff like general experience that may lead to an entirely innocent accident, say misjudging someones intentions on a roundabout and pulling out in front of them or something. The sort of thing you're more likely to misjudge in your first year of solo driving, even if that's when you're 17 or 27. Random example but the point being it's not entirely down to young person = chav = 100mph = crash. That's why new driver insurance is always fairly expensive even if you're older.