Because the "HR-system" has a high-salary bonus payoff in many cases (think of headhunters who get paid as % of the hire's pay). HR people also work on a "prize" mindset, the word "headhunter" says it all. Getting someone out of a FAANG into your team is highly valued, independently of early performance indicators. The whole IT team thing also has a social-status and peer-pressure game on, so the next hire may ask "who will I'll be working with?". Finally, hiring is imperfect, it's human nature almost all the way and it suffers from a lot of guesswork, due to lack of hard data on the candidate, scarcity and critical time pressure on the process. So hiring managers are biased and tend to project an image of who they are hiring based on prejudices and a rigid mindset.

It needs to be pointed out that, although an ex-googler may not be as productive as our imaginary highly productive Kenyan engineer, they could bring valuable experience with things such latest/secretive tech, hot contacts in the business with the resulting network effects, great communication skills and novel and healthy work habits and team culture.