I'm guessing most people who say the emergency fund should be in the market are under 40.

I say this because it's been over 10 years now since the last recession and a lot of folks under 30 haven't experienced one since they've been out on their own. They haven't experienced a prolonged bear market. Folks over 40 can remember the tech wreck (2001-2003ish) the housing bubble implosion (2008-2012ish). Folks over about 55 can recall how bad the late 70s, early 80s were.

And then there's just stuff that's not related to the economy. Life happens. Emergency appendectomies. A family member gets diagnosed with cancer. You burn out on your profession. The longer you live the more of this kind of stuff you've experienced.