If the parent isn't being ordered home yet by the government, state law says "Your child is too young to leave home alone while you work" and they shut the schools down under circumstances where you will find it nigh impossible to get daycare at all because the entire city is dealing with the same issues, this could get you fired (or charged with child neglect if you go to work and leave your child home alone).
Lots of people are amazingly callous, but some aren't. Some are reluctant to pull the trigger on a decision like that, knowing how many people will get caught in the crossfire and have their lives essentially ruined even if we magically come up with a cure for the virus tomorrow.