Maybe because In the US the myth of 'The American Dream' is so prevalent and instilled in people's mind that social mobility, or the lack thereof, is not worth discussing? Also, social mobility is linked to income inequality. Income inequality being high in the US, talking about social mobility would lead to a discussion about income inequality that is not possible because people are too attached to a low-taxed society? In the UK people expect more of the government, including helping the lower classes? But I'd say this US/UK distinction is rather a US/Europe one. If you want to experience the American Dream, go and live in Sweden/Denmark.