Computing this depends on estimating/forecasting/extrapolating/guessing a lot of values. E.g. what kind of revenue growth the company will have. How the structure of company expenses may change. Parameters like a discount rate make a huge difference in the estimated value. In many cases the majority of the value for a discounted cash flow valuation of a stock comes from the tail term where you give up trying to unroll the contribution for each year and make a simpler approximation of what the company will be doing 20 years+ in the future.
The computed share value has a lot of sensitivity to adjusting some of these inputs, and in many cases it is not at all obvious what values parameters should be set to.
Many of these are permanent consequences that will absolutely effect earnings 15-20 years in the future. The market seems to be pricing in an assumption that this will be a blip: we'll reopen, businesses will rehire, and 2021 will look much like 2019. I don't believe that's likely.
Stocks are based on expectations of future profits, i.e. psychology. "In the long run, the stock market is a weighing machine. In the short run, it's a voting machine."
Few investors' psychology will let them look at a year of bad news and still think "Oh, it's going to get better in the future." After about 3 months you start doubting yourself and wondering if maybe you were wrong in the first place, and you've entered a brave new world where people randomly die and commerce or long-term plans are impossible. All of the economic data - corporate earnings, employment, share prices, etc. - will reflect the new normal, so there's no reason (other than your memory of what the '10s were like) to believe that share prices would always go up.