The nature of a lot of engineering (bog standard business logic and UI implementation) is such that a good engineer can adapt to shifting needs and focuses. For example, an engineering team can weather a shift from consumer to enterprise much more easily than sales/marketing. But non-engineering positions are also more monetarily and professionally rewarding in the case of success - a textbook example of high risk, high reward.

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.