Most of them are not the best and brightest. The best and brightest get scooped up locally. The people who move are usually just good enough, but they're hungry and willing to work their asses off. That is their edge.
Sources needed. For example, the hottest tech co. paying $1m comp packages, OpenAI. Is leadership from the Bay Area? No, it was the brightest people, from St. Louis, North Dakota, Albania / Canada, Russia / Israel. You can take a look at all the top tech co's and top brass in the Bay Area, it will be like 80% transplants. The brightest didn't start the next best tech co in Missouri or Albania.
Sources needed as to whether they were the brightest people in their home towns / countries when they left or if they became brighter through hard work and learning after arriving to the Bay Area.
Even if they were the brightest, are the thousands of people working at Bay Area companies the brightest from back home? Having worked with quite a few of them, I assure you that there are plenty of brighter people where they come from.