From what I have seen. I think it’s fairly frequently that an EM runs out of people to put up for promo based purely on merit. When that happens they reach out for folks with tenure in the team. After all, big part of EM’s own promo packet is how many of their reports did they promote.

Another thing is that folks who stick around long enough end up acquiring almost archeological knowledge of systems involved which can be a strong incentive to retain them, for example by awarding a promotion even if they don’t tick many of the “Staff boxes”.

Besides that, I have seen attrition being another cause for these types of promotions. When a Staff TL leaves, it will create a void that an organization often fills by promoting the next most tenured Senior engineer.

As always, FAANGs are huge orgs and these generalizations do not reflect any and all experiences folks might have.