Yeah but those exceptions are pretty big. The hiring pipelines at FAANG are so over-tuned for false negatives that there are plenty of engineers who are good enough to work at a FAANG for FAANG-level compensation yet are not actually getting offers from FAANG companies. They're on the market for (market minus FAANG)-rate compensation.
Then you have to remember that FAANG companies are large enterprises, by definition, and that comes with a lot of overhead - design by committee, politicking between middle-management fiefdoms, not being a part of the conversation when irrefutable directives are issued by executives four levels above you, varying levels of paperwork and documentation that are necessary in large organizations. That's soul-sucking for a lot of people, and those people will exclude themselves from FAANG-level compensation, and are on the market for (market minus FAANG)-rate compensation.
The real reason why a lot of founders can't hire at market rate compensation is that any early employee, even if you're paying them market rate, needs to buy into your vision just as much as you do. The upside for early employees, even more than potential compensation, is in being a strong influence, including at relatively senior levels, as the company grows. If, as an IC, you find yourself being recruited by somebody who you think is a strong and experienced leader, selling a product that you personally think is important, then you grab the bull by the horns and get on. If somebody who rambles and can't make eye contact asks you to join to build out Uber-for-pidgeons, it doesn't really matter how much compensation is being offered; you're going to walk away.