As others have said, MONEY FOR THE TAXMAN is what it's all about. Remember they were hinting at closing the pisshouses pubs when the schools went back? That would have a drastic effect on the economy, but parties at home much less so.
Perhaps also, they're thinking that there would be fewer parties anyway while the kids are at school. I'm sure this has been planned for a while.
They're probably also taking the view (and actually, I think they're right) that with organised activities such as sport, people are more likely to follow precautions such as hand gel, not sharing equipment, and so on; the group organisers don't want to be shut down for not following the roolz, and there's more accountability; whereas this is not the case in private houses.