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.
I have to imagine this priority shift is in part due to the money markets being what they are.
In these cases, companies raise funds at the same valuation as their previous round, often labeled as Series A+ or Series C+ or Series B Extension.
Another, less common strategy involves using a SAFE (Simple Agreement for Future Equity), which will convert to equity during the next priced round.
But it was never officially 'worth' that much in the same way as a market cap of a public company anyway. If they do a downround, where they raise money at a lower valuation than the previous one, that's generally bad for everyone, so there's a strong tendency to try to 'wait it out' and just pretend they're still worth $1B and hope the market recovers and no one has to write down their investments.