The longer you're at a big company, the more you get used to it, and the harder it is when it's time to move on. I worked at a big SV company for ~4 years, did extremely well, stack ranked in the top 5%, got raises, my last review I got a "critical talent stock award" -- it's easy when that's happening to imagine yourself there forever.
Then over about 6 months my division was divested, my manager was replaced, and a bunch of us in the old regime got laid off at once. It was heartbreaking, I was in shock. I bought my first house there, had 2 kids as an employee. Transitioned really to a fully-formed adult there. The work was interesting and I had a wonderful team. The job was a part of me, and I went from blessed top performer to the soup line in a matter of months.
But that's what happens when you don't manage your career really hard. If I had been smart/ruthless, I would have moved diagonally to a new company and started making a name before they had the chance to make a move on me. But I fell for the siren song (and vesting schedule) of the Big Corp. This isn't an unusual story.