Not sure if this applies to everyone's question, but I remember hearing something back when I started to the effect of "you don't actually get added to the seniority list until after you complete your check ride".

Seems like health reasons to me. I think the last guidance was that we are supposed to limit gatherings to 10 people or less, having 60 people stuck in a classroom doesn't seem like a great idea for potential health reasons. Especially when people are flying in from other places, all interfacing and spending all day every day together. What happens if one of those people all the sudden starts showing symptoms? At that point you would have no way of knowing how many people has been exposed. Probably has a pretty low chance of happening, but I sure wouldn't want that on my conscience if classes full of people, instructors, etc were all exposed all of the sudden because I made the call to keep training people.