This is also related to how engineering is often viewed (correctly or not) as a young man's game. The most commonly cited reason for that is how there's always new stuff to learn, but even if there wasn't so much churn in the relevant knowledge, the fact that younger employees can grow and "catch up" professionally much faster than in other business functions plays a role.
The problem is that you're a known quantity, and the founders/investors see you as a great worker -- in a certain role. They perceive some problem, and since you're part of the system with the problem, you are seen as incapable of fixing the problem. Better to hire Magic Mike from Facebook or Brilliant Bob from Google to come in and make everything better.
I've seen this happen so many times. If you're an engineer, your choices are generally a) swallow your pride, ride out your vest and don't advance in your career, or b) quit, lose the money, and hopefully move into that more senior role somewhere else, so that you can be Magic Mike next time.
The idea that just hiring a team lead from Amagoofacesoft will solve all your engineering management problems is deeply embedded in the Valley ("nobody ever got fired for going with IBM"). Never mind that you'll be scraping the bottom of the middle management barrel at those companies, since the really good ones aren't going to leave.
Startup engineering is a young man's game in part because it's harder to fool more experienced engineers into drinking the koolade.