> the reality is all grey area. All of it. There are very few black and white answers

The inherent contradiction in this quote needed to be highlighted. In fact "grey thinking" is superior to black and white thinking in many cases, just not all. In some cases the answer is just simply yes or no, and no buts.

As to why grey thinking is so hard I think the article leaves out a major reason: grey thinking is more demanding. We will as humans by necessity always try and simplify things as far as it is possible, in order to save mental resources. Sometimes it is good enough, many (most?) times it leads astray, or at least gives an impression of simplicity that isn't really there.