I think peer pressure might be a considerable factor. You screw up on the anchor line at the surface in rough seas, you forgot to turn on your tank, you are over-exerted and you just took 2-3 half breaths of seawater.... No body wants to be the wuss who ditched his belt on the surface, but this sounds like a situation when you better make a decision FAST (get that snorkel to work or ditch the lead).
I've never ditched a belt, but I have found over 30 of them. Most were probably lost accidentally and I bet many of the divers were embarassed by loosing the belt. Loosing dive gear ain't what the cool kids do. There has got to be some peer pressure to not lose the belt.
Obviously people shouldn't be sheding lead like dandruff (especially if deep or people are below them) but I've seen a number of incidents where people should have dropped lead and didn't.
In fact, when I was a DM on a boat, I would have to commonly ask divers to remove their weight belt between dives. Often I might see a large man still wearing a 12 lb belt, dive boots and a bathing suit on the surface interval. When I would suggest that they might be more "comfortable" with the belt off, it would be obvious that they forgot they were wearing it.
I was always worried some fat, out of shape tourist would fall overboard when we were running and by the time they realized they were sinking, their ear drums would be crushed and they would be confused why kicking of their feet (in dive boots) wasn't getting them to the surface from 35 feet.
If people are stupid enough to forget they are walking around a boat with an anchor tied to their waist, I always wondered if they are smart enough to drop it in an emergency. The option of dropping lead needs to be TRAINED into divers so that it comes to mind quickly in an emergency.