Theoretically, why would a young person invest in the market rather than waiting for a downturn before pouring money in?

I know this question seems dumb, and frankly, I know there’s an answer, but I’m not smart enough to see it, so I appreciate us all recognizing my ignorance ahead of time.

We all know markets are cyclical, we just can’t guess exactly when the market will boom or bust (and thus we get the ill-advised game of trying to time the market). Right now we’re consistently hitting all time highs in the market, with the most recent major downturns being 2008 and COVID.

Theoretically, at least at some point in the next 10-15 years there will be a downturn of some kind, and probably at least one very strong downturn within the next 20-25 years.

My question is: If a young person seeking long-term gains via an ETF was looking to invest, why wouldn’t they be better off allocating their money into a savings account and then pouring that money into, say, VOO, at the next major downturn rather than buying at all-time high prices?

Our theoretical young person would have time for those gains to recover after the downturn, would theoretically have access to a higher profit margin since the buy-in cost would be lower, and they’d have that money on the side if they ever needed it for an emergency rather than locked into the market.

I suppose I‘m assuming a downturn will occur (but how often do we go 20 years without a noticeable downturn?) and that current valuations will be higher than wherever the theoretical downturn bottoms out which is what makes the wait worthwhile, but I’d appreciate any help from people here much smarter than me. I know time in the market rather than timing the market is king, so I’m trying to wrap my head around this.