Interesting question; nothing "just happens," there are trends and cause-and-effect that make it interesting to figure out why things are, like for example, why blue collar characters are underrepresented in comic book superheroes.
Someone once pointed out in COMIC BOOK NATION that the DC Heroes of the 1950s possessed a "serve and protect" mindset, personifying ultimate authority figures. For that reason the characters of the 1950s were either policemen or wise scientists.
One possible reason for the fact blue collar characters are underrepresented is, there is a degree of admiration for intelligence and education that is only usually found in two other genres, detective and science fiction. One thing that can't be said about comics is that they are anti-intellectual. Look at all the scientist characters: Peter Parker, Reed Richards, the Black Panther, Barry Allen, Ray Palmer, Batman (at least the sciences related to crimesolving), Bruce Banner, Hank Pym, and so on.
Superheroes' primary function is to serve as wish-fulfillment projection, which is why socket wrench heroes may be uncommon. How many superheroes for example, are outright MONARCHS, royalty of foreign lands? Aquaman, the Black Panther, Namor, Wonder Woman, Geo-Force, Amethyst: Princess of Gemworld, Thor (son of Odin).
Plus, from a storyteller's point of view, it's easier for a roving reporter to go AWOL for a few hours and fight crime than it would be for someone on a factory production line. What's he going to do, say, "I'd love to save the city from destruction, but my shift's not over for another two hours!"
I think you've hit it on the nail here: one of the most important criteria for superhero jobs is that they have a degree of
mobility. This is why so many heroes are reporters, actors, private detectives, and millionaire playboys; characters that have jobs founded in a specific location are usually the exception. Stories can be advanced when, say, Clark Kent is sent to cover a mysterious explosion in the Metropolis Mine Works or when Simon Williams is asked to make a movie in the American Southwest (where dinosaurs have been discovered).
Here's one working class hero for you: Underdog, in his guise as Shoe-Shine Boy. "He's both humble, and loveable."
The ultimate "working class hero," naturally, is the always put-upon everyman Spider-Man.
While he may not be working collar or have a job of any kind, Captain Planet certainly has working class hair: check out his bright green mullet!
Green Arrow started off rich but went bust. For a lot of time it was unclear whether he had any job at all. I think he was doing something by the time of Mike Grell's series, but it might have been no more than delivering flowers for Dinah's shop.
Minor detail: Green Arrow was a reporter for a while after his company had disappeared.