People who succeed don't understand or know why. They might have earned their success (some did, some didn't) but being competent at something is orthogonal to knowing why one succeeded at it, over the hundreds of others who fell on their faces. They know the least about how the system works, because they won the lottery by having the whole edifice stay out of their way.
On the other hand, people who failed tend to know exactly why they failed. This doesn't mean they understand the system either. Individually, almost no one does--successful or otherwise. Thing is, no one wants to listen to them. All they can tell you is what not to do, through a story with a depressing ending. It's very easy to write them off as bitter old washouts who never belonged in the game in the first place.
In general, there's a conflict in our society that, I think, explains a lot of the weird dichotomies that emerge, including political ones, and it's this: holding an external locus of control and a negative view of society makes you correct, but isn't always conducive to ideal mental health. On the other hand, believing in an internal locus of control and holding a pleasant view of society, although it largely makes you incorrect, seems to increase the likelihood that you will enjoy positive mental health and therefore succeed. Classical decision theory holds that having more information leads to better decisions and higher utility; on the other hand, when it comes to locus-of-control and the nature of society, being correct is observably maladaptive. It's tough to say which is "better".