Because I don't like having a bottle cap dangling in my face when drinking, so it will be ripped off. Of course that has a 50% chance of leaving the tether attached to the rim so now you can stab yourself in the face while drinking. Now you have to rip/twist that off, too. Of course that bit can't be reattached to the bottle any more ...so into the natural microplastic cycle it goes. If they could at least fix the predetermine breaking point at the rim ... but that would require a design by people with a brain.