Why do some patents have well behaved and successful children, while other parents raise failures?

A lot of people like to blame the parents when that's only a fraction of the issue. I think bad parenting has become more of a lame excuse. It might work when they are preschoolers, but as they slowly become adults it doesn't ride anymore. Ultimately every individual is responsible for their own successes and happiness. You can set a kid up to succeed, but it's up to them to take the reigns. And on the other end of the spectrum, I've personally had bad parents and know many people that come from homes that were full of neglect and abuse yet they are the most well mannered, successful, and polite people I've ever known. They aren't without issues or perfect by any means but if we can over come that.. why cannot others overcome their situations?