The trades are not promoted to students from middle-class families because one of the biggest markers of middle-class status, in our culture, is having a college degree and working at a job that requires such a degree. They most certainly ARE promoted to students from working-class families. Guidance counselors at schools in low-income areas routinely steer students toward the trades. Politicians talk all the time about how great trade schools and certificate programs at community colleges are; but only rarely do they send their own children to anything other than a selective four-year college.
In many cases, working-class parents, observing this, also encourage their children to attend a four-year college. This might or might not be the right choice for any given child, but it’s an entirely rational choice. College degrees, and occupations that require them, are generally regarded as higher-status than the various alternatives, even when those alternatives pay well. Maybe that isn’t how it should be, but it’s how it is, and most people want to maximize their children’s status. Having a college degree means, at the very least, that you can attend parties where everyone else has a degree and not feel out of place, and that kind of thing matters when you’re up for promotion to manager. It also means that you simply have more options in life; people who went to college CAN always go back and learn a trade, but they’re also able to apply for jobs that do require a degree – and a lot of jobs require one not because it’s absolutely necessary to do the job, but essentially as a filtering mechanism. To some extent those employers are filtering for real stuff, since it takes a certain baseline of intelligence to make it through college, as well as the ability to plan ahead and more or less stick to that plan; but they’re also engaging in social signaling: saying, essentially, “this job is for the type of people who go to college.”
Class privilege sucks, but it’s not going away, and it does have a significant impact on the course of people’s lives.