Without being aware of the details, in my opinion one problem is that housing is being built in areas that were previously green fields and often without much thought being given to pedestrians. If you can chose between chancing it across a major highway or walking a three-mile detour in the middle of the night to use the nearest legal crossing, I think a lot of people will take their chances, especially at times of day that there isn't much traffic, which typically also coincide with times they may have had a glass too much to drink (which may be the reason they aren't driving in the first place) and as a result of that misestimate the speed of approaching cars.
So quite possibly, if you provide safe and legal crossing places (as in, over or under passes) and combine that with fencing then people will use them. But some areas have far too few of those and its always difficult to put them in retroactively .
So people being dumb may be one part of the story, but lack of legal and safe crossings may be another.