People who whine about gear are, as others have said here, compensating. They're also failing to realize that tools are specialized, and that cameras are specialized tools. I wouldn't expect the same results from my Fuji XT2 as my Canham 8x10 Wood Field, because they're very different tools for very different purposes.
As to the remark about not knowing the shortcomings of your camera, car, or house before buying it, well, you might know something about the car or the house or the camera is not as big/fast/whatever when you buy it, but you don't think it is a problem at the time. You buy a house with two bedrooms when it's just you and the spouse, and the small size isn't a problem. Two children later, and what was cozy is now cramped. You bought the sports car when you only had to drive on weekends. Then you changed your job and now commute 50 miles a day, and the fuel economy that was known to be sub-optimal but not a problem is now a major problem. Et Cetera, et cetera.