It isn't easy to just recruit extra teachers. We have a teacher recruitment problem in any case, and it would take time to train people up. If there was a way of encouraging teachers who have left the profession back into the classroom, then that might work, but I can't see anyone queuing up to teach in a situation which is clearly putting the teachers' health at risk - why would they?!
Classrooms aren't big enough to accommodate 60 kids. They're not even big enough to accommodate 30 comfortably in many cases. The learning will have to be done online, or schools will have to adopt a blended approach.
This is what should have been done in the first place tbh. It was entirely predictable from the start that there would be significant staff shortages if schools went back full time with no real possibly for social distancing, so I totally agree that they should have seen this coming. I just don't think it's that easy to magic up more teachers on demand, so better efforts should have been made to avoid getting into this situation in the first place.