I wish I could say it is the only reason, but this is just one of the reasons why almost all brain imaging studies are bunk. Any “signal” that rises above background really tells you nothing, since in many cases it could be that the vast majority of activity is occurring in parts of the brain obscured by the ever-present background “noise”. Of course I am vastly oversimplifying for the sake of explanatory clarity but for anyone interested in the fine detail and the actual explanation see Robert Shulman’s Brain Imaging: What It Can (and Cannot) Tell Us About Consciousnes, 2013.