One thing that you're also missing here is that (IIRC) employees of one of the colluding companies that applied for a job at another colluding company would be

One thing that you're also missing here is that (IIRC) employees of one of the colluding companies that applied for a job at another colluding company would be 'reported' and some may have been fired for shopping around for a job elsewhere. (E.g.) A Google employee sends his/her resume to Apple, then Apple tells Google that said employee is looking for work elsewhere.