Quite possibly because the stock market is no longer reflective of the health of the US economy. It appears that the majority of stocks (80% or more?) are held by large entities and institutions, not individual investors, so it may reflect large troughs of money, very large businesses, not small businesses, which are not on the markets. Also, private equity investing has increased over the decades, as those with a lot of wealth want to find somewhere to put that money while avoiding the regulated public investment arena. Individual investors don't get any view or ability to invest in private equity opportunities, as they aren't listed, and there is no avenue for them to invest in. I suspect private equity will continue to grow as a whole, and public equity decrease, and as that continues, the public markets will no longer (and don't now) really reflect most of the US. So what may be keeping the markets at highs now is that they are disconnected from small businesses, so they may not at all see any real damage from COVID-19 in the short term. In the long term, these large businesses rely on individuals to purchase, and at some point I expect the inability of individuals to buy products and services to have an impact on those larger companies. No idea if that will come to pass, but without customers, it's hard to see how these companies can sustain earnings in the future. It could also be that a lot of people are jumping in expecting a V shaped recovery, and the markets are looking like I 'V' now, but I don't think that applies to everything outside of the markets, which are not likely to make a 'V' shaped recovery -- how and if the two interact -- "Main Street" and "The Markets" is anyone's guess at this point.
Again, no idea what happens next, as the behavior of markets is changed by the changes in who owns stocks, in that we have large institutional investors rather than small individual investors, and it's very difficult to forecast the behaviors of complex adaptive systems.
This article goes into some of the potential reasons for market behavior today:
https://www.nytimes.com/interactive/2020/05/26/magazine/stoc...
I will close by saying that there will be much less public ownership of equity within companies in general, as less companies will be listed in the markets, so over time, large companies/businesses will be privately owned. Whether this is good or bad, I don't know, but it cuts out regular individual investors who can't even invest via indexed style funds, because that to my knowledge doesn't exist w/r to private equity funds.