Ask HN: What's the incentive for 'Founding Engineers'?

I'm trying to understand the incentive for engineers to join a startup as a 'founding engineer'. To me it seems like taking on all the risk that a founder takes (pre-pmf, pre-revenue, etc.) but getting a small fraction of the equity that a founder gets. The risk-reward ratio doesn't seem to add up.

Looking for insights from folks who have been in the startup world.