First, it wouldn't be an apples to apples comparison because the Americans without insurance are not randomly selected, so you'd be carving out a population with other confounding variables. And second, if doing so showed better outcomes here than elsewhere, we'd be saying "we have the best care, but 10% of the country doesn't get to have it. Tough luck!". Third, our overall spending is so much higher, we'd be admitting that we spend way more money to not even cover everyone.