Google and many other tech companies and the investors that ultimately control them fear losing control of their business, in this case an extremely lucrative near-monopoly, to their employees, especially the technical employees. Thus they prefer young, relatively ignorant, naive, hard-working, compliant, inevitably more error-prone employees to more experienced employees, despite the apparent cost. It is a matter of political and economic control.

Thus they have an unwritten rule to minimize the use of employees with more than ten years of hands-on technical experience. They will employ more experienced employees if they absolutely must, but otherwise they avoid it. Hence, there are some but very few technical employees with more than ten years of experience. The numbers trail off with increasing experience and age. There are also exceptions for more experienced employees from usually wealthy families affiliated with the investors.

It is an irrational and hidden class system. It is costly. It is not fair. It almost certainly makes it very difficult to make technical advances and breakthroughs that require more experience, the wisdom of age, etc.