Unless you're implying a set of companies are all in league with each other to reduce churn/competition for talent
They have been caught doing so in the past. [0]
[0]: https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...
What they need is that somebody who passed the interview believed it was a lucky break and would not tempt fate trying a similar interview at another company. If they considered the interview just a filter, they probably would not be so lenient with letting people to apply many times and even encouraging this.
This is a proven reality, and has been out in the open for a long time.
This was 10 years ago and involved 8 companies.
Concluding from one crime that they all do it, and that's just what those people are like, you can't trust them etc, is an impulse we all do well to resist.
Of course, concluding that nothing like this will ever happen again is also naive. The incentive to cooperate is certainly still there.
My impression is that since the 2010 case, wages have gone up dramatically in that class of companies. There are also more big companies with owners who aren't pals from before. I think. Just guessing, relly.