This question, of course, presupposes that the fallacy is in fact common, but some support for this is given in The Bounds of Cognition, Chapter 6, and in earlier posts here on the blog. So, let's go with that. But, if the fallacy so common, and as some have noted, obviously fallacious, then why do so many philosophers commit the fallacy? I think there may be many reasons, but here is one.
Compare the burden of proof involved in the simple coupling arguments, on the one hand, versus the burden of proof in cognitive equivalence arguments, on the other. It is much easier to show that something in the body or environment causally influences what goes on in the mind than it is to show that some transcranial or transcorporeal process is the same as some intracranial cognitive process. There are very few instances in the extended cognition literature where one finds side-by-side comparisons of putative cognitive equivalence like those found in the three modes of Tetris play and in the Inga-Otto case. Maybe these are the only such comparisons. Moreover, these sorts of comparisons almost invariably invite observations of dissimilarities. Such observations are standard among critics of extended cognition, but there appear to be no cases in which a critic denies that the claim that one or another thing in the body or environment causally influences the mind. So, one can see that advocates of extended cognition will regularly be tempted to claim that causation is sufficient for extended cognition, since it is easy to find causal influences on cognition. If causation were sufficient for extended cognition, the case for extended cognition could more easily be made