Even if you believe that "kids don't get sick from COVID", you can't ignore that teachers do. And many of them are in high-risk groups -- older, with comorbidities. Most are vaccinated, but they're still taking a risk.
Teachers are already smarting over the fact that (in the US) they're literally trained to take a bullet for their kids. So when you tell them to open the schools "for the kids", they're hearing that they're disposable human beings, and they don't like it.
Open the schools incautiously, and there won't be any schools to open. Because a school without teachers is just a warehouse.