Your point is valid but I think your figure is high.
There's a handful of states you'd need to have a permit which you can't get right away.
There's also the cost. Depending on where you look anywhere from 30% to 75% of Americans live paycheck to paycheck or are one large unexpected expense away from catastrophe. They likely don't have the expendable income to go spends hundreds on a firearm at any given moment.
This is why, even though there's somewhere between 300-500 million civilian owned firearms in the US, those firearms are owned by roughly 32% of the adult population in the country.