Couple things....
1. Blue-collar union jobs (that Democrats pretend to support) have been largely chased out of the country. For the last 40 years anytime the Democrats had to choose between trade unions and the Greenies, they sided with the Greenies every time.
2. Unions successfully lobbied the government to regulate things that used to be the unions' job. This took the heavy lifting off the unions but then many workers don't see a need to join a union.
3. There is a practical limit to how much money a private sector business can pay its employees. I used to own a small business and have personal experience with this. I was limited to how much I could pay my employees (and myself) by how much my customers would pay me. Makes for a lot of work at the negotiating table for both sides.
4. There is no practical limit for paying govt employees. The unions donate work and money to elect politicians who will give them what they want and there is no real opposition. State and local governments will simply raise taxes and the Federal government can simply print more money. Hence, low-hanging fruit.
Also when I was a young college student I spent a summer working in a steel mill so I have personal experience with what
@sgtsandman mentioned.
When I got back to college my grades were a lot better the next year. Paid well but I
didn't want to do that the rest of my life. Which turned out to be an unfounded fear because within five years most of the steel jobs in western PA disappeared.
BTW, I have a degree in economics, an MBA, taught macro economics, micro economics, marketing, and labor-management relations on a part-time basis at different colleges for 20 years, have a partial writing credit for a college economics textbook, and ran my own small business for 21 years writing real paychecks for real people and real checks to the government for real taxes every month and every quarter. I describe myself as a
highly-educated redneck.