Notice how that link is not specific about what counts as upper class and middle class. It is not the case that people in the "lower" classes decided not to ge

Notice how that link is not specific about what counts as upper class and middle class. It is not the case that people in the "lower" classes decided not to get any skills. It is that the economy changes over time and their skills are no longer compensated well and, in general, US society is not good at accommodating people to change their skill-sets to move into a new career. You might have anecdotes about people who made such a career change, but for society as a whole, it is too difficult.