When does it make sense to leave big tech?

What sorts of reasons make people leave big tech? I recently received an offer from a tech start-up is super interested in my candidacy and they're moving what they can to try to match compensation. The move includes a promotion. It also seems like I'd have access to career growth and learning due to people there wanting to work with me to help me succeed.

However, no matter what they do, they won't be able to match the compensation I'd receive within a few years, assuming I grow into a senior engineer role and stay at a big tech company. I see driven, career-focused individuals moving out of FAANG-like companies to start-ups and non-FAANG companies, so I'm curious: what drives people to do so? When is this the right career move?