Historically it was considered a beneficial necessity to gather the children to write down knowledge so that it could be brought back home for the whole family to learn from, but in the age of the internet perhaps separating children and parents is never good at any (young-ish) age?
Since the advent of air-conditioning, there really isn't any good reason to close schools during the summer. But, like the internet bit before, we've just never bothered to stop and actually think about what we're doing. We carry on with the status quo simply because that's what we did in the past. Not because it makes sense, just because that's what we do.
But in establishing subsidized daycare now, we don't have to think about the time before air-conditioning was invented. We only have to worry about the constraints we have today. Hot summers are not a practical problem as of right now.