In addition to the staleness argument (which is somewhat countered by its inverse: the good, helpful commit message might be hidden under a huge layer of naming

In addition to the staleness argument (which is somewhat countered by its inverse: the good, helpful commit message might be hidden under a huge layer of naming convention changes and the like), there's also the off by default. A comment always helps itself to some mental bandwidth, whereas the commit is nicely tidied away unless you ask for it.