The problem is bigger than just having robust union representation (which I fully support). The problem is the concentration of high paying, high skilled jobs in the same handful of places.
Used to be manufacturing was done all over the country, and it was easier to standardize a union wage for work say building cars whether you were in Flint or Buffalo or Gary. Nowadays if you're in high tech, biotech, finance, energy, medical research, etc -most likely you're in a cluster with a bunch of other people in the same field. That gives you bargaining power, because you can easily get a job at a competitor without having to relocate and those industries aren't trying to complete on cost of labor.
If you're not in one of these places, and you work in a one industry or one company region, you have little bargaining power. A union in this case would help you achieve a livable wage, but would struggle to get you to earn as much as your high flying counterparts who are being fought over by multiple companies in the same place. This is the problem of stagnating wages, which also prevents a remedy for income disparity. If Kep gets canned by Citigroup in NYC, he can take his act to JP Morgan. If Kep gets canned in Toledo from the one car manufacturing plant left, where does he go? Nowhere, so most likely he puts up with miniscule pay increases because he's happy to just have a job.