Only anecdotal, but I have observed as much over the past few decades. It's hard to be certain why as there are so many factors at play, but I suspect that a part of it may be due to how we perceive certain jobs. There's a certain stigma with most traditionally "trade" jobs like electricians and plumbers. Many of these type jobs actually make more, especially if you own your own business or have a fee structure that is tied to your actual work.
I've known journalists, biologists, programmers, middle managers, government employees, and many white collar jobs to scoff at or look down on other jobs. In some conversations I have reminded these same people that the filthy guy in line they are making fun of might be pulling more than twice their salary, and often may still be university educated or have needed to go through some rigorous training. I am not sure what makes some people think they are above certain jobs or better than other people.
The Obama administration sentiment that everyone needs to go to college has done no favors helping certain classes of jobs. White collar jobs or being a celebrity, athlete, or famous in some other way is seen as success, while anything else regardless of quality of life, salary, long-term prospects, independence, and so on is seen as failure, second-class, or even undesirable. I respect anyone who works hard, and especially those that do things that I can't do. I need these people to survive or at least have a better quality first-world life, therefore I always make sure to treat them well. It helps when you come from a culture that considers it rude to not at least offer to feed your plumber or make a coffee.