Businessmen are in the business of making money. If their "assumption" was wrong, someone would have noticed it and published articles and such saying, "Hey guys, the way to make money really is to hire people with sloppy resumes". Then it would have caught on that not hiring people with sloppy resumes doesn't actually make you more money. That hasn't happened and there is a very good reason for it. They aren't wrong. While there are exceptions to the rule, they are not common enough to make it worth the risk.
Besides being human and fallible, businessmen are also fairly egocentric, as a group, and may often assume they know things as facts when in fact they are things everyone takes for granted without scrutiny. Some even go so far as to think they know better than the data they get from even the best studies.
Examples:
1) Before the advent of the personal computer, several CEOs of computer building companies were openly skeptical that anyone would want a computer for personal use.
2) Coke thought they could replace the original formula with New Coke
3) Ford thought they'd only pay out a few thousand dollars per wrongful death claims resulting from collisions involving Pintos.
4) companies keep trying rotating shift scheduling despite it being a known productivity killer AND a major factor in decreasing workplace safety, as well as being correlate with dozens of ailments- including heart disease and diabetes- as shown by decades of workplace studies.