(Obviously "schools" are not one thing, there are thousands of school districts under all kinds of political jurisdictions, but generally...)
Aside from physical mitigation issues (ventilation, etc.), people just flat out ignore the unsolvable personnel issues that schools face daily when staff/teachers are getting covid and there isn't exactly a large pool of willing substitutes. There has been a lot of wishing away of the health risk to children, and a lot of just ignoring the issue of spread (in school and at home), but even if "we" accept those things, the very practical issue of making sure there are sufficient people watching your 8-year-olds every day is unsolved and often unsolvable!
I'm not saying, "close the schools!" I'm saying that, well, you try being a principal of one right now.