> I don't know why manual work has been so denigrated over the last century.

As a farmer, it is funny to see how people react to you based on the current profitability winds. When farming is a money maker, everyone acts envious and treats you like a king. When times are tough, they think you're a slack-jawed yokel.

I expect in that lies the answer to your question: We denigrate anything that isn't, as a rule, making a lot of money. Manual jobs generally haven't made much money in the last century, and humorously the exceptions, like professional athlete, get exempted from being considered manual work.