1) More capable of starting their own, competing business. Even if the old guys won't be eating google's lunch, they have money and connections to defect and start a consulting business with their expert insider knowledge of SEO/self-driving cars/whatever they were working on.

2) The greater breadth of knowledge they have about industry and their personal preferences means they're less likely to be shuffled quietly into a menial role that makes the company the best ROI.

3) Stronger personal definitions of morality. Need a morally questionable piece of code written? If you ask a young enough person they likely won't question the morality but if you ask an old enough person I could imagine them refusing.

I think hiring young people has it's own risks, but they seem to be generally further down the line.

Losing control can happen at many scales and to many degrees.

I would expect many hiring managers at large companies to be intimately familiar with the different risks associated with different age employees, consequently discriminating in their profession. Short of some damning leaked documents or conversations however, I can't imagine how any would ever get convicted.

Playing the devil advocate here I bet internally they have metrics telling them there isn't any value that people above 30 add compared to 20 year old straight out of college.