Nobody likes carrying around heavy things. I guess warm-water only resort divers dont have to suffer that much if theyre just shuffling to the end of the boat. But if a diver knows that they dont really need to carry around all that extra weight, thats pretty much all the incentive they need. So they just need proper training.
As I mentioned before, my OW instructor overweighted me in training -- but he told me I was overweighted and gave me tips and incentive to continually drop weight, which I did. And he taught me (mostly) mid-water, not on my knees. That was enough for me. Some of the instructors here go much further, helping new divers achieve optimal weight from the get go, which is commendable.
But even if the instructor hadnt provided this information, I read the materials and knew about the concept of proper weighing, and how to do a weight check. The manual even covered things like breathing, skulling, and proper emptying of a BC, all things that can lead to overweighing. Took me awhile to put the theory into successful practice -- l longer than my OW dives, but soon enough I did it.
It seems like you need a combination of poor instruction and a complacent diver to result in overweighing. I'm sure that there are plenty of divers overweighed by 4-10 lbs bc of such a combination.
So it seems like the answer is just proper instruction - midwater, off their knees
And if a diver still ends up in the extreme case of being 20+ lbs overweight -- well, you cant fix everything, and folks like this are why they have the Darwin awards