FTFY.They're trained poorly because heavy and knees are the easiest for the instructors, or at least they think it is. . .
I spent a couple of weeks this summer helping out a friend's dive shop with a bunch of open water students at a Boy Scout camp.
My friend and his instructors teach students on their knees but I teach from go in neutral buoyancy and trim. We split the groups up by instructors, so we each had between 4 and 6 students each.
It was interesting to compare how the different groups performed.
It took my group much longer to finish up with their initial skills (partial flood, reg recovery etc.) and get to swimming around - maybe 30 more minutes of work. But the gap closed as we progressed through the dives.
Because I was teaching for my friend's shop, and keeping to his timeline, I didn't teach exactly how I would have, but in the end I had most of my students hovering horizontally and there were a few who were back kicking.
It is faster and easier to glue your students to the bottom of the pool, but in my experience, well-trained divers dive more, take more classes, and spend more money on equipment.
I don't necessarily teach open water students in a complete DIR rig, but I will say one thing about training with the long hose. Multiple S drills make students much more confident and competent divers. I've had 2 OW students this summer who had the same problem, broken zip ties on their primary mouthpiece that resulted in a primary floating away with the mouthpiece still in their mouth.
Most recently, this happened in open water dive 1. I glanced away from my student to reposition, and when I looked back, she was holding her primary second stage in her right hand and the mouthpiece was in her mouth. As I had one of those "Oh ****" moments we instructors love, and move to donate my primary, she calmly removed the mouthpiece from her mouth, and switched to her short hose. No panic, just a little confusion as to what happened.
As far as recreational students not wanting to learn DIR style diving (what @boulderjohn mentioned) almost all students do not know what they do not know. When I marketed neutral buoyancy and trim no one really cared. When I started talking about small classes and personalized service, things took off. Once students get to open water, they can see what they've paid for, but before they get there, they don't understand.