In other words, researchers cut corners. You should never assume that something is a certain way without rigorously proving it. How did these papers make it past peer review?
Another thing is that statistical analysis is kind of the missing piece in many researches training. It maybe not be "researchers cut corners", but they might not know some of the prerequirements for the analyses used. Even in this year, 2016 when I talked with a colleague in my institute I found out he didn't know a single assumption for t-test, who is a senior postdoc and used it everyday.
Papers merely employing some method are rarely reviewed by the same peers as those of the methods papers, and often neither aware of or state the assumptions of the method.
This is how this happens, at least in neuroscience. You can look down on it and judge, but neuroscience has not really found its Kuhnian paradigm yet, so be nice.