> what's to stop you from being in that 1%?
You wrote this like it were a rhetorical question, but let's talk about social mobility. Wikipedia[1] will tell you that it's "movement of individuals... between social strata." Surely you'll agree that "the 1%" is a prime example of a social stratum, and that becoming a member of that stratum is a clear movement.
Obstacles lie between them.
To overcome these obstacles one requires capital: economic, social and cultural.
If you start without them, you'll not go far. Physics are relentless. It may be impossible — within one lifetime — to navigate the socially constructed maze from third-generation-diabetic-of-color to business services firm partner, despite the two persons being physically proximal!
Yes, one can and must make decisions in this life, and if one has free will then by all means, enjoy the faculty, but one is not free to purchase items that aren't in store. One is not free to pull levers that aren't available.
You might ask "why do poor folk fall prey to predatory lenders? Why not invest that windfall in a CD, or a more appropriate financial vehicle?" But that question only exposes your naïveté: some folk have simply never heard of the phrase "financial vehicle". They lack the cultural capital to even see the social constructs allegedly available to them. They are literally blind to them.
Free will does not enable one to open invisible doors.
Apologies for this long-winded tirade: your rhetorical question struck me as absurdly naive. You claim you "can name several people who have overcome those impediments," but "several" is not a statistically significant number, and will not convince a rational person that the United States has high social mobility.
What's to stop one from being in that 1%? Massive, invisible, entrenched social constructs requiring immense luck or capital to navigate. And then your "free" will — certainly not free of biases, neuroses or addictions — can still fuck it up.
1. https://en.wikipedia.org/wiki/Social_mobility