The biggest problem is that it's classed as a civil issue, not a crime.
I steal from my employer: they call cops, I go to jail. Very simple.
My employer steals from my paycheck: I have to call a lawyer, they have to agree to take my case, we have to sue, employer produces fake payroll documents, I better have proof of what my real pay should have been, we have to go to mediation, figures are exchanged, employer claims they can't pay the full amount without going bankrupt, have to take a settlement, lawyer takes at least a third of that. Big pain in the ass, no guarantee you'll see a dime. Even if employer pays you back, they can go right back to screwing the rest of their employees over.