Because Windows can view and extract them out of the box without installing any additional applications. If it supported anything better out of the box I'd guess people would use that instead.

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lvh 34 minutes ago | parent [-]

"The operating system makes it easy to mess with" doesn't seem like a particularly useful property for application file formats.

	▲	TeMPOraL 27 minutes ago \| parent [-]
		It was, back when software development was run by hackers and not suits and security people. Easy access was a feature for users, too; back in those days, software was a tool that worked on data, it didn't try to own the data.