Because if you fire them they will turn against you. They are your employees, and their wrongdoong will be attributed to you. They might very well agree to test

Because if you fire them they will turn against you. They are your employees, and their wrongdoong will be attributed to you. They might very well agree to testify against you, civilly or criminally, in exchange for the suit being dropped. But if you keep them on the payroll (and often pay their lawyers) you can create a common front and limit their ability to 'go rogue' and harm you. The reckoning can come after the suit's settled.