7.5% of the bottom 20% make it to the top 20%. In a perfectly fair and equitable society, you'd expect it to be 20%. I wouldn't call this "quite rare," though I would call this uncommon.
But what percentage of the bottom 10% do you think make it to the top 10%? It's probably even fewer.
And to go step further: I'm not convinced the "top 10%" actually has everything they need to become a billionaire. Zuckerberg's parents were both doctors, Bill Gate's dad co-founded a successful law firm, Musk's parents owned an emerald mine in South Africa. These folks almost certainly grew up in the top 5% of household income.
If you need to grow up as a child in a top 5% household to "have everything you need" to start a wildly successful company, what percentage of the bottom 5% make it to the top 5%?
I'll bet you it's indeed "quite rare."
Is 7.5% “quite rare”? That’s around (and between) the portion of the population who is left handed and the portion of the US population that has naturally blond hair, neither of which would I describe as “quite rare”.