Mark Powell explains, without the intent of offending anyone, that divers do stupid things because they are stupid.
If you take a moment to examine most diving accidents, as pointed out in the post referred to here, it is easy to understand why you would have to agree.
As we all know, there can be any number of factors that contribute to diving incidents. Mark points out in an article as well as the following video how easy it is, even for smart divers, to do stupid things while diving.
The main point being made is that diving would be magnitudes of order safer if we could force all divers to just follow their training. Naturally, everyone would agree with this statement. However, Mark goes on to point out, in the following excerpt, that hundreds of divers do things daily that do not align with their dive training. Herein lies the problem.
If you look at some of the recent diving incidents that have occurred such as the tragic double fatality of a father and son who died while cave diving on Christmas day, or the diver who refused to analyse his gas an ended up breathing 100% oxygen at 30m or the rebreather diver who jumped in with oxygen, diluent and handsets all turned off then you can see why I might say that. The British Sub Aqua Club publish a summary of diving incidents every year and a brief glance at this will show that dives do a whole range of stupid things. Not only that but they do the same stupid things over and over again. Most of the mistakes made each year are the same as the mistakes made the previous year. The short answer is that divers do not follow their training. It would be very easy to stop the vast majority of diving accidents if we could just force divers to follow their training.
Watch the following video recorded at TekDiveUSA in Florida. This video contains many insightful, almost entertaining, examples of how easy it is to make mistakes and /or formulate inappropriate conclusions that result in bad decisions.