system
A chat on subreddit /r/NoStupidQuestions
human
I always figured it was linked to a time when children would spend the summer holiday working on the farms.

No idea if that's the real reason though.
gpt
That's it.  When school was becoming widespread, kids helping during summers was not really negotiable.

Now, continuing the break seems to give more benefits than detriments, and summer seems the best time for it.