You either have no idea how the engineering job market works or are being deliberately obtuse.
These companies set the price for engineering hires throughout the valley. If they collude to keep salaries down, all other companies vying for engineering talent will not have to pay the market rate. In effect, they are guaranteeing themselves access to engineers at depressed wages by market manipulation.
And in the real world, you retain talent by paying for it. It doesn't work for free. If the talent is so freaking important, then pay for it. It will stay if you are paying as much or more than is available on the outside. If you are a business, and you have critical folks, you should be paying them like they are critical, not limiting their job options.
You would care about this if you were the talent in question, getting underpaid and reported for applying to another job. Imagine how many careers have been derailed in the valley by these HR departments reporting job seekers to their employers.
You would also care about this if you were a shareholder or current employee at one of the cartel companies, given that all who have applied for a job from one of these cartel companies to the other and been rejected now are a pain free class action waiting to happen. All who have had their wages depressed are a class action waiting to happen.
And in the end, everyone loses except the CxOs with their golden parachutes and the Blood Sucking Lawyers.
That's why this is fscking stupid and wrong. You can resume speculating about their motives, as though that means a gd damn thing.