It's an interesting one, isn't it? In my opinion, it boils down to those people simply not having either the time, energy, or mental capacity to stop and actually think for a second. It's generally a combination of #3 and one of the other 2...

By design, of course.

They're either too poor to worry about anything but bills and food, too distracted by themselves/sports/work (or all 3) to put effort into anything else, or just not intelligent enough to actually grasp the required concepts. And you can't forget the healthy dose of constant revolving door propaganda to keep them constantly mindfucked.