In terms of not applying what they were taught and did in class. In class they may have been put in a BC and trimmed out in it, and even assisted with that some. But after class they go to a boat and rent a (different) BC at the dock. They get asked how much lead, or the op guesses, and that lead is put in the integrated waist dump pockets. If they ask if some shouldn't go higher they learn the rental BC has no trim pockets, or they let lead fall out so the op doesn't like using them, or only with big weights, of which they have only a few choices. So they dive like a sea horse. And repeat at the next boat, which leaves diving in trim far in the distant past.
And the notion of them diving the BC by fin skulling to keep level is not anything they experienced in class, when they dove a rig whose weight was distributed to put them in trim. And sculling to keep level takes constant effort, which is tiring and not something they signed up for on vacation.
I haven't rented BCs recently, but my experience around others and at boats suggests this is very common. Their experience and understanding of what went into diving in trim may not be strong enough to cause them to insist on a BC and pockets that they can distribute weights in so they are in trim. So they aren't. If they got taught it, the notion of trim was a nice one, but not worth fighting the dive op outfitting divers for. Nor is it clear they would know all the options of how to attach lead, nor brought add on pockets or bungee with them for that. So sea horse.
(I flew with my BP/W, spare pockets, and trim lead.)
A couple of points here. The first is dispelling the myth, that the equipment, (BP/W, or BCD with trim pockets) is crucial to goo trim. It is not. Trim is a function of body positioning, HOWEVER a buoyancy compensater that enables you to distribute weighting to assist in making it easier
When teaching in my jacket I routinely dive overweight, having enough to hand out if students become buoyant (say positive tank combine with a trigger for the fight or flight reflex) and enough to ensure if I dump I can control or restrain a uncontrolled ascent from a student. I can also (and have) lashed 2 AL 80's in addition to my back gas, to a jacket, to make a tech dive. it wasn't' pretty but I could still perform it without compromising the dive.
Being overweight doesn't prevent me from holding trim, nor from good buoyancy as a stop (although it does take concentration to control the bubble) but that is a factor of experience.
I will certainly concede that divers have an addiction to weight, the majority see it as a comfort blanket and pile it on (others strive to get to the absolute minimum even though it isn't required - I was that person)
My good friend who works a live-aboard sees this regularly, she sees divers overweight but they've convinced themselves they need it. You take it off them, they fret underwater, trigger teh flight or flight, become buoyant and think they've proved a point.
His solution is some "placebo" weights, she's mocked them up from fiberglass and slides them on to the divers weight belt replacing lead. the diver doesn't' notice and performs the dive without incident. She continues this over the course of the week and helps them with their trim, until they're sorted. Only then does she fess up.
Now I was that diver. During my OW course and for the following 10 days of diving , I was continually corking. I was given all sorts of (correct ) advice and help by my instructor, but as a new diver I knew better and turned to lead. I was a mess. if we stopped to look at anything I would have to keep moving. I got myself out of this rut, by reading and practicing and learning from my peers.
Not all divers want to go that way, and they refuse to be given advice. They know "what works" and are not prepared to accept they need less weight. They get to see pretty fish and that's all that matters.
A bit like me and my golf swing. I got around the course, who cares if it was a 99 or a 85?