My guess is they figure it will make little difference due to network effects, and by making this exception they can remove a meaningful source of political resistance to the change. If the vast majority of users are running operating systems that declare underage status, then websites and apps will be able to use those signals to sidestep legal liability for actually doing anything meaningful to protect underage users, and new laws can be passed that assume the use of those signals is possible. That it isn't 100% doesn't change that very much.