It's also pretty silly to say "Well if companies A, B, and C are hiring individually, the workers can all choose to work for A and B but not C. But if they all contract out to company X, then company X workers need to shut up and either work for all three or none. They can't even ask for things to change, they can only quit."
Again, you want to pick your own job, work for yourself or find the right employer.
Picking what you work on is not a reasonable expectation of any employee.
If nobody is doing C, start your own firm and get rich doing something no one is doing. Or perhaps you'll find out why no one is doing C. But forcing your employer to make bad business decisions under the guise of "working conditions" is a bridge too far.
Doing it your way can actually cause much worse business decisions. If employees don't give any verbal feedback, only quitting, then the employer might take a hot-button contract and suddenly have multiple important workers quit all at once, losing far more money to replace them than the contract was worth.
Unions have exactly zero with choosing to do the work you want to do while getting paid for someone else.