Simply put, they can get away with it.
Public sector payrolls are available to the voters if they're interested in seeing them.
Private companies will pay as little, and cut as much as, they can get away with since no one is looking except their accountants.
Plain old greed. It's the companies themselves that have been known to reach out to poorer countries and make deals, bringing those immigrants in at lesser pay.
Because they can. Because no one stops them. They've done a great job convincing us to blame the immigrant for coming and taking our job, and not the corporation for treating labour like line items on a spreadsheet.