It's not that inequality is a problem, but if you are willing to go 50 miles from the coasts you will find people in the trades who do OK, some with help from a union, some not. Some of them get paid pretty well but what they all have in common is that they didn't start their career with $200,000 in college debt.
So many things make it invisible, however, including:
* a heavy involvement of illegal aliens in "services", the "trades" and "agriculture" in places like California. These people work hard at being "invisible" because if they become visible they get deported; a result of it is that people don't have a realistic image of this work and in the people that do it
* the media is demographically unrepresentative of the country as a whole: if you read the whole New York Times (particular the Sunday metro) it seems almost as provincial as a paper from small town South Dakota. There is a lot of content that plays to the fears and anxieties of over-represented groups in the New York City suburbs.
* for HN in particular: every kind of engineering other than "software engineering" requires occupational licensing and has professional associations that both stick up for workers and set a bar for training and quality. As a result if you work in a place like a library or a hospital you'll never quite be accepted or taken seriously by the licensed occupation folks who run the organization. To people in our field, the idea of having a group that looks out for our interests seems unthinkable, which is great news for all the other people who are organized and have one less pressure group to fight against. Thus the idea that you could have a union job is just as invisible as having an ag job.