> I'm no business genius, but you'd assume that investors are.

Absolutely not a reasonable assumption at all. Most people are not very good at their jobs.

> What's the steelman argument here?

If you're right about your prediction, but wrong about the timing, then you're wrong. The markets can stay irrational longer than you can stay solvent. Meaning, even if you know it's going to crash eventually, you don't know if it's tomorrow or in three years. Betting on stocks going down is extremely risky -- you have to get the timing pretty much exactly right, or you lose everything (potentially much more than you put down, depending on how you do it). Meanwhile, bull positions are basically free money until this point. Getting completely out of the market means you lose to inflation, although many investors have taken this strategy anyway.