You can’t bring a new business or new hires up to speed instantly. Not only are the lockdown months already lost but there’s a ramp up period. Further, some of the companies in the index likely won’t survive. Even if they will be replaced down the road that shouldn’t be reflected in the prices of currently existing companies.
For example, infrastructure is still intact, demand is theoretically still there just suppressed, capacity to produce is still theoretically there.
Unemployment is based on employers short to medium term outlook. I.e. Can I pay this person for a month and will the person be a net positive?
So the two measures differ and in weird situations like now we see how much they differ.