Because the TSA is so inconsistent is the reason why. Sometimes they demand one method, sometimes the other. So you do both in order to minimize your interaction with them.

Small cases (revolvers, for instance) that you can pack in a larger case, get one inside the hard case, and one inside the larger soft case in which it is packed.

Large cases (shotguns, rifles) are pretty obviously gun cases, there is no disguising it. One goes inside, one gets taped to the outside.

However, even this precaution won't save you from the other means of TSA incompetence and inability to comply with FARs on firearm transportation. Such as failing to screen the cases at the check-in desk, and requesting that you give them the keys so they can search it out of your control, which is the most common one.