There isn't any evidence, it isn't actually true. Debt is inescapable in a country where a trip in an ambulance can wipe out a family's entire income. It's just a cultural prejudice born from the American idolization of free markets (which itself is a vestigial expression of European classism and belief in noble divinity), rugged individualism (and a pathological fear of societal bonds) and the Protestant work ethic, mixed with a bit of racism and xenophobia.