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Back in the late 1800's, jeans were designed to be work pants.  Men would complain the pockets would rip when they stowed their tools in the pockets.  A new design was formulated with rivets on the top corners of the pockets, making them stronger and able to hold tools without ripping.  Now you know.

I'm guessing, zippers came along when men kept wetting their pants trying to get those damn buttons undone.