“Just” doesn’t mean what you seem to think it does. Schools have very little margin for soaking up absence at that scale. If one out of five teachers are out, the kids will not be doing the learning which is the stated justification for not keeping them safe at home. Teaching is hard work, not hitting play on a video, and it requires time to prepare in addition to classroom time – teachers don’t call in sick for fun, it actually creates more work for them since they still need to create the lessons and deal with the impact on their work when they’re back, and in many districts a decline in test scores will cost them money (in some, so does taking a sick day).

School administrators are extremely busy with all of the extra work they’ve been given, and if they’re babysitting students that work (often required by law) will not happen.

But, at this point, they are pretty much babysitting them, at least according to the account from the reddit poster. Everyone's talking more about the testing than anything else with little added benefit.

Yes, note the large number of absences — that was basically the point: schools which have been resource-starved for many years don’t have anywhere near that much slack capacity, especially since one of the sources of substitutes are retired teachers who are at higher risk.