The best argument I've been able to find is when they're offered a company match in their 401(k) or similar.

Basically - total market index funds have very low cost, and the entire market has very rarely gone down 50%, and eventually recovered.

Since your company is matching you 50% (or 100%) on your contributions, if the market goes down you're losing "house money/free money" you wouldn't have otherwise.

Then after years and years they just get used to it.