I have no idea why you think I.Q. is a primary factor. What I have found here on this forum is that most farmers and ranchers are very intelligent and experienced in many things. They rely on their own abilities and intellect to overcome all sorts of problems that they encounter on a daily basis.
First, accidents do not happen due to one factor. (See Malcolm Gladwell, Outliers regarding airplane crashes) Many look for the one thing that caused an accident when in reality, it was a combination of things. Fatigue, distractions, poor maintenance, poor communication, etc. There are typically several things that combined, create an accident.
In what I have seen in industry, tribal knowledge and peer pressure play a role in whether or not accidents happen as well. I have seen companies roll out new safety requirements that always result in resistance. Most accidents are avoidable but only when people follow the rules. This isn't an issue of a high or low I.Q. It may be that they were in too big of a hurry or they have done something so many times, that the safety rules do not apply to them and others follow their lead. (I worked in a factory of 600 where accidents happened on average 1 time/week. This dropped to 1 time/year with a large push to develop and use any and all safety practices and devices with a zero-tolerance for ignoring the rules).
With all that said, the answer to your question has many answers. Farm equipment is dangerous, many people work alone, and bad habits transfer from one generation to the next. People get tired because they work hard and equipment fails.
And capitalism is far from perfect but it's been better than anything else man has tried so far.