Politics. I found that getting promoted within FAANG depends largely on your ability to navigate politics, not on how good you are at your job. You will move up much faster if you optimize for "Find the right people and get them, through whatever means, to say great things about you at performance reviews" - I've seen many brilliant people at the company who never moved up because they were bad at navigating or manipulating human relationships. And I've also seen people who were, let's say, not the brightest, but extremely good at manipulating people and make themselves look good - they moved up very quickly.

It has also given me a new appreciation of why startups can be so efficient. The joke that every gmail redesign made the product worse but had to be done because someone needed to get promoted is no joke. When politics falls away and everyone's incentive is to become profitable or to build a good product the result can be magical.