Knowing that most varroa stay hidden in cells, I can see how your point makes perfect sense, because the powdered sugar coats the bees and varroa are removed during cleaning. If this is the case, then why are so many beekeepers using this method?
Because one does not use powdered sugar at just any time, but instead when the hive is brood-less. Either taking advantage of queens which pause laying during a dearth, or doing a small split out with the queen so most of the hive is queenless. When there are no larvae and no capped cells, female mites must have something to feed on, so they are forced onto the adult bees. Then powdered sugar dusting is used to induce grooming, knocking a much larger percentage of mites out of the hive.
Although a couple of studies have "shown" powdered sugar ineffective, they have not been done properly. Of course dusting a queenright colony full of capped brood is not effective! Most of the mites are hiding away where nothing, not even oxalic acid, can touch them. Of course dusting on a solid bottom board is not effective! Mites fall to the bottom, then climb back on to the next bee that passes.
Powdered sugar does not kill mites, and it is not a long term solution to varroa any more than organic acid or organophosphates. In the long run the only useful method of control is to select for bees that can resist and survive mites and the diseases they vector.
But for a hobbyist without the resources to take the crushing losses that follow when treatment stops, sugar dusting is a cheap, low-tech method of varroa control that can help hives survive. For what it's worth, sugar dusting has been repeatedly demonstrated in studies not to harm the bees and not to induce resistance (since it's mechanical and not chemical in it's mode of action). Whereas oxalic acid weakens colonies somewhat and treated colonies have the same winter failure rate as control colonies (from Nanetti 2003).http://scientificbeekeeping.com/powdered-sugar-dusting
Here's part 3 of a research review of sugar dusting. Not well organized, but about 2/3 of the way down there is an interesting note that one of the authors in the FL study which "debunked" sugar dusting responded to a letter by saying that while sugar dusting was ineffective "within the parameters of the study" (which dusted queenright colonies with capped brood), that she has seen it work in the real world and would recommend continuing to use it as part of IPM methods until "the science catches up".