First, that's not true. National Geographic's yellow border design is trademarked, which means they at least care that nobody else has a magazine with the yellow border. I would guess that Time's red border is also trademarked, but I'm not sure. Either way, you don't see any other magazine with a red or yellow border. For books, I'd guess certain series like the "for Dummies" series might have a trademarkable design; at the very least, book cover designs are copyrightable, although that hardly seems useful given that nobody else would publish the same title with the same author...but I'm not too sure you could get away with publishing a self-help book with a yellow cover, a chalkboard, and the title written in pseudo-handwriting.

But besides obvious legal issues, it's just best practice to have a design language that matches your use case. For example, Voat is just a clone of reddit so it makes sense they look the same. But you wouldn't, say, design wellsfargo.com in blue, or design chase.com with a wagon logo or other Old West imagery.