Valuations have "fallen" but not actually fallen: there are very bad consequences to raising what's known as a "down-round"(1) so no one is actually doing that unless they are absolutely forced to. So no company is interested in actually allowing an investment at that lower valuation. They only do that if they are absolutely forced to, because they desperately need the money.

So while yes, when the valuations go down seems like a perfect time to buy (buy low, sell high!), in these closed markets it is difficult to find someone to accept your money when it is down. This is a big difference from the publicly traded market, where you can essentially always buy stock. But in these private markets, everyone agrees that the value of a share of company X is lower than before, but no one is willing to sell you a share today at that price, so you can't actually invest your money.

1: Where the top-line valuation is below the previous valuation. This is extremely bad for a company because investors almost always have protections for a down-round, so the loss generally is felt entirely by the workers and the founder.