This is in general a really bad idea. Not only can you be on the hook if they make a mistake with the install, or mess something up like venting that ends up damaging appliances or plumbing, but if one of them falls off your roof, depending on the circumstances you could be personally liable for their possibly life long medical treatment (or end up in an extended lawsuit as they try to claim that you personally directed their work). Make sure your contractors have insurance.
Insurance and overhead (eg. safety harass) exist for a reason other than to drain your wallet. Roofing is also a physically difficult job. You won't find many 50+ year old to couch on a rooftop all day, regardless of pays.
How are children supposed to know the adults in authority positions around them are full of shit?
I think that was my underlying point. It isn't blame, its just that they don't know any better. They've been sold the One True Path To Adulthood and Success and Happiness and it is a false bill of goods.