It happens in technology too. I'm not sure how many people remember Dragon Dictate, but they were well ahead of the game in voice recognition in the 90s and 00s.
Then they made a bad mistake and got into a marketing partnership with individuals they would have been better off avoiding. This not only killed their business, it also meant they no longer own their own IP, which is currently wasting away in business limbo.
That's the point of the OP. Grit will only get you so far.
It's not about individual hero stories, it's about the probability of success for equivalent effort for entire populations, and how vulnerable they are to the effects of a single bad mistake.
That's where you can see the negative effects of some backgrounds, and the positive effects of others, most clearly.
The assumption behind this belief set is that they were the select few who could do this. There is an alternative hypothesis that out of the large number of people with poor background who did this, they were the few somewhat randomly selected to succeed.
Certainly if you look at their biographies there seems to be a consistent element of right person in right place at right time - which suggests that there might be a lot more 'right people' than get lucky.
I.e. nobody is going to get lucky by watching TV.