If contracts are violated then the impacted parties can seek redress through the courts. Government employee unions aren't needed for that.

You have to have your contract violated for a significant amount before you can notionally afford to hire a lawyer to fight it out. Below 5 figures it doesn't make much financial sense to do that for most people, so they just eat it instead. It's how a lot of "theft of wages" and other mistreatment happens so often. Lawyers don't take those cases for free, and court isn't free either. And you're not going to instantly appear at the top of the docket for something small like that especially if the government buries you in procedure. They can do that for years.

But sure, yeah you can seek redress through the courts.

Suing the federal government solo is an insurmountable task for most people -- even more so while they're being constructively terminated. Employee unions have been suing on their workers behalf over the past year, but the executive branch can drag out federal trials for a lot longer than people can stay without a job.