The jacket is a paper sleeve, not part of the binding
A hardcover’s dust jacket is designed to wrap around the book and come off again. Its front and back panels cover the boards, while folded flaps tuck inside the covers. That makes it useful for displaying the title and artwork—and easy to remove if it gets damaged—but leaves it less anchored than the book’s sewn or glued binding.
The jacket also has to bend at the folds where it turns around the covers. Those creases help it fit, but they are natural weak points: repeated bending can leave a sharper crease or eventually split the paper.
A crowded bag turns small movements into wear
Inside a bag, the book shifts as the bag swings or its contents settle. A jacket edge or flap can snag on a zipper, notebook, or another book; friction then drags the paper while the heavier hardcover keeps moving. Pressure from other objects can add dents and bends. The hard boards can take much of that contact without obvious damage, while the thinner jacket records it as scuffs, wrinkles, or tears.
Publishers use jackets because they provide a printable surface for cover art, information, and promotional copy without printing all of that directly on the hardcover boards. The tradeoff is that this removable paper layer is exposed to the very handling the boards are built to withstand. In a tightly packed bag, the jacket can show the trip’s wear even when the book itself comes out nearly unmarked.
