Having been offered positions at a number of companies who "really need" someone with my skillset, a lot of these places are shockingly inflexible on pay. I make 3-4x what (I infer) they normally pay people in nominally similar positions, and they seem absolutely unable to match that for procedural reasons, even though based on their description that seems like a reasonable investment for a specialist they really really need. The companies I end up working for are typically outsize successful, and I attribute a lot of that to their willingness to fork over some extra cash when it makes rational sense, and shockingly few companies in SV fit this description.
Having been in a form of ea myself for around 10 years I'd also add that it takes a long time to start being effective. Lots of skills besides tech to learn, which will be very specific to whichever form of altruism you're involved with. So although I haven't looked at those job postings, the simultaneous over and under supply could possibly be explained by skills mismatch.
These roles are just like many others in the field, companies are supposedly desperate to fill them, but set unrealistically high expectations for the hiring bar so the role is never filled.
Context: 6 years professional experience, open to moving anywhere in US, experience at two SV unicorns, fine taking a reasonable pay cut to work on these things, still rejected or never heard back from every job of this type I've applied to/interviewed for.