Because they are portraying it as if the only options, at least outside of shotguns, are prohibitively expensive and incredibly overkill. You do not need a (minimum) 800$ weapon designed to hunt large game to do the job. You do not need to completely destroy all traces of yourself, a la McNutt, to do the job.
In something like this you want to be very sure.
And this shilling of overkill weapons does nothing but make people unsure and uncertain about things which they should be very sure and confident in. And it's even lead people to becoming unsure of weapons that previously, everyone had been in agreement were the 100% guaranteed option that should eliminate any uncertainty. But with all the fearmongering, eventually even the guns everyone could agree would do the trick became "is this really adequate enough?"
You don't want to take any chances being left a half alive vegetable for years, or lying on the ground conscious but unable to move being eaten by animals.
And, if you aim properly, you won't.
This line of thinking does not take into account that some people do not have access to whatever ridiculously overpowered weapon they want, and needlessly sows doubt into their mind, as well as simply sowing the seeds of doubt into everyone in general. It is, once again, portraying anything short of the most powerful firearms on the market as being inadequate and thus having a high risk of failure should they be tried, which is blatantly false. I have noted previously that about 90% of handgun suicides are successful, and I am willing to bet that most of those are people who used common caliber handguns (9mm, .45, .40, .380, probably even .22) rather than the BFGs being suggested here as of late.
The insistence to shill the overpowered guns, and disregarding anything less as inadequate and likely to fail, is nothing more than needless fearmongering that serves nothing except to make people unreasonably scared.
How about we just tell everyone that unless one shoots themselves directly in the brainstem with an anti-tank rifle, they're going to survive and live as a vegetable for the rest of their years, and that literally anything less will just result in failure? Since, y'know, overkill is better and anything less than overkill will result in failure.
9mm is often considered somewhat marginal in police work. There have been numerous incidents where police have shot criminals multiple times with a 9mm and yet they are still fighting. I remember a video showing - not a person but a pit bull - shot 15 times with 9mm before it was stopped.
Those who consider 9mm inadequate are usually fudds and their opinions are typically subject to ridicule. And even if they weren't, what do they suggest? I'm willing to bet something along the lines of .45 ACP, .38 Special, .357 Magnum or 10mm, around that ballpark. Hardly the monster calibers being thrown around recently.
Also, when it comes to self-defense training, people are typically trained to aim center-mass, not at the head. We are talking about shots to the chest here, not a case where someone is aiming a weapon at their brainstem point-blank.
Adrenaline and/or drug use can account for most cases where people continue to fight after having been shot.
Also, very important point: The police and military likes 9mm handguns because they have high capacity, multiple shots. We don't have the luxury of multiple shots, just one.
And because they are lethal enough.