why would an engineer work for a fraction of a percent of equity and a below market salary when they can take a job at FANG?
Once you hit a few million in the bank, have a house, priorities kind of shift. Not for everyone, but for those that would work elsewhere for reasons not money.
The problem you'll find (I've found) in this case is that management in many of these "startups" expect a certain kind of ... ahem... motivational/authoritarian structure ... that lacks effectiveness or sense with someone who has paid off their mortgage and is mainly there in order to ship things and enjoy crafting software.
Put it another way, there are people in every company whose reasons being there can conflict with the motivations of an engineer with the priorities you describe. Often those people end up being your manager.
Doesn't have to be a startup. Can be something _not your own_ but aligned with your interests. Maybe you care about the environment, or you're fascinated with weapons and don't particularly like people or whatever.
Once you hit a few million in the bank, have a house, priorities kind of shift. Not for everyone, but for those that would work elsewhere for reasons not money.