This is an interesting point.anyone playing D&D is going to assume polytheism anyway. Anyone with even a passing familiarity with fantasy genre fiction is going to assume that. It's something of an outlier to see monotheistic fantasy fiction.
My familiarity with fantasy fiction is rather narrow - some "modern classic" children's authors (Susan Cooper, Alan Garner), the Earthsea stories, JRRT, REH and a smattering of Vance. Of those authors, JRRT assumes monotheism; Earthsea really does seem to be "godless" (the God-kings clearly are just humans, and the ancient beings like the Terennon and the Nameless Ones of Atuan are more like primal spirits in D&D terms); REH has many demon-worshipping cults and the like, but what few manifestations there are of what might potentially be divine action (eg in the Phoenix on the Sword and the Hour of the Dragon) are consistent with monotheism. Vance I think is godless. The British authors like Susan Cooper and Alan Garner tend to present Celtic myth (and pre-Christian European myth more generally) as still active in the world, but it is not polytheism in the Egyptian, Greek or Norse sense - a cosmos governed by a pantheon of divine beings. It is more like the spirits and magic of "nature" - actually similar in some respects to Le Guin's nameless ones, but benign rather than malevolent.
These influences probably shape the way I think about religion in fantasy RPGing. I have run plenty of polytheistic gameworlds, but have no trouble either conceiving of and running faux-mediaeval ones.