I know the cool thing to say here is that folders just make more sense, as we don't need a 7" SEAL knife to open a box or perform similar daily tasks that actually happen on a regular basis. And I agree with that too, but I'm going to also agree-ish with the OP. And I believe, in re-reading his post, he is specifically talking about the use of knives for self-defense/protection, and he's asking why he keeps seeing folders as the poster knives when this specific use of knives is talked about/advertised/etc. I don't know that I have personally experienced what he's talking about there, but if he has, he's got a point.
Again, he's not asking why would anyone carry a folder if where you live, you're allowed to carry a nice sized fixed....he's asking why use a folder instead of a fixed for self defense, or situations where self defense is a distinct possibility. I totally agree with this, and yes, I even have a personal story to go along with my opinion! About a year and half ago, I was taking a walk thru the neighborhood with my family - lovely Wife & our 5 kids - and our neighborhood is a rural foothills town in No. CA btw. I NEVER take such family walks without a fixed blade (and honestly, I'm one of those dorks who prefer EDC-ing a small to medium-small fixed anyhow...& where I live there is no length limit at all as long as it's open carry). Not so much because of bears or mountain lions or even crazy mountain-men, all of which we have in our area, it's really for dogs (lots of very protective dogs in my rural area, as the people up here tend to prefer privacy & discourage "visitors") & because I feel cool & think I look pretty cool with one of my favorite fixed blades on my hip. So, back to the story, we're walking along, nice sunny late morning day, and all of the sudden a very aggressive pit bull runs up to us, stopping just about 10 feet short and growling & barking like crazy (we were walking in a residential street BTW, on the opposite side of the one lane little street from the house this dog ran from...a dog that always runs up and barks at all cars, including us, when they drive past her house). I immediately put my arms out stopping any of my family from getting in front of me & "asked" my wife to get in front of the kids, which was already doing....I don't recall doing it, but I got my knife into my hand & ready to use...the dog's owner comes out after about a good solid minute of terror, and says "don't worry, she won't bite ya". 3 seconds after she says that, her dog charges at me. Luckily, I was mentally prepared for it, kinda, & put my shoulder into her as she kind of half jump-lunged at me...I was able to deflect her head off to the side of me w/my left shoulder & plunged the relatively long bird & trout (4.5" cutting edge, about 9" OAL) I was walking with, into the dog's neck...right above where her shoulders start. Because of my crazy high levels of whatever chemicals course thru one's body in such times, the entire 4.5" penetrated this very powerfully built, all-muscle dog...& even though the bird & trout was only about 1" wide at it's very widest point & not the very best choice for protection really - but I was carrying it because I just bought it & wanted to wear it...it was enough to prevent the dog from engaging any further. She let out a heartbreaking cry of pain/defeat, & shuffled back to her house, into the arms of her owner. This woman was already in full tears & screaming "he killed my baby, he killed my baby!" The dog ended up surviving btw, & she doesn't seem to be impaired physically, though she does act much differently...PTSD is the best way I can describe it. Her owner, in case you're wondering, never once offered her apologies....nor has she said anything negative to me or my wife...just nothing....& I understand her love for her dog, I do not understand how she seems to be cool with her dog nearly killing children, which she of course would have done if she could have.
Anyhow, this story got longer than I wanted, sorry guys, but I guess my point is, or one of them anyhow; the OP has a point, when you look at the precise wording of his post...I certainly prefer fixed (though I prefer 3" - 5" edges mostly) over folding for self-defense situations (& all situations, as again, I'm one of those annoying fixed blade lovers), which are exceedingly rare....to the point of most of us here on BF will go our entire lives without ever once having to defend ourselves/loved ones in such a manner. But I did, & I often wonder how differently that situation would have turned out had I only been carrying a folder or no knife at all. (my oldest boy, 13yo at the time, had a fixed and a hickory walking stick on his person too BTW...so we try to be prepared, as this wasn't the 1st time a dog had barked at us)