Curious: Why do private schools have such long holidays?

[Erebus]

Genuine question- I mean, I know why state schools hols are what they are, more or less (helping with harvests... etc) but why the 8 or 9 weeks, for example, in private? How did that come about? There must be an historic reason seeing as they all pretty much do the same thing, with 17 weeks holiday a year.

Private school in the early 90s: 8w in summer, the usual week-long half terms, and 4w each for Easter and Christmas. But it was an academically selective school, so we got through the work so much more quickly.