I have to concur that most of the "This lens is soft" threads I've read is the result of individuals with OCD. In those threads that the OP supplies pictures, preferably with EXIF, in nearly every example I've looked at in this forum and others, typically, the issue is 'operator error', not equipment fault.
Perhaps the 3 biggest 'soft' causes are:
1)the camera focused on some point other than where the photographer 'wanted' it to focus,
2)shutter speeds in the 1/60th and slower range resulting in a slight blur due to subject movement,
3)shooting wide open with an f2.8 or faster lens with the subject at 10 feet giving a too-thin DOF.
Other photographers are expecting $3,000 lens image quality using a $200 'kit' lens.
A variation of the $200 kit lens is someone with high end gear trying to distinguish 1mm lines from 500 yards away with a 400mm lens...totally unrealistic expectations, if you ask me.
Then there's the 24"x48" print from a 5mp camera JPG output that isn't "razor sharp".
Or the too-often maligned Canon 24-105 f4L as 'soft' because it's a 'kit' lens. OK, when I had a 60D, my 24-105 was fully sharp enough, and only by pixel peeping would I see a bit of softness. But after performing MFA on it once I got a 5Diii, I think it'll stand up to comparison with the 70-200 f2.8L mark I.
And, as Roger wrote and previously linked to above, every now and then, there really IS a clinker in the pile. And just how many 'perfect' cars get manufactured every day?
Lastly, back in the "good old days" of 35mm film photography, we were literally 'overjoyed' with anything that was accurately focused. And in those days, the film plane had some 'wiggle room', and the film itself could be a bit 'wiggly' in the frame, as well. It seems to me that everyone today is expecting Hubble Telescope quality for everything they shoot with their $400 camera kit.