The great thing about medieval Europe is that it's different enough from modern day Europe to not be insulting while being familiar enough to have common tropes. Throw in the fact that there was a mish-mash of religions until relatively recently. Recently enough that we've inherited the myths, long enough ago we don't have large portions of society still worshipping.
So in my home campaign I can base it on Norse religions. Published campaign worlds also largely use a Greco-Roman style religion as a basis with a pantheon of gods. Scrub the name off the old gods (or just steal them outright cough*Tyr*cough) and you're good to go.
Let's say I wanted to do an oriental culture. Awesome. Well ... hmm. Now we have the first problem. What do we do about religion? Assuming Christianity and Islam are no-goes we have Buddhism, Confuscism, Daoism. Religions that have been around for thousands of years and are still being practiced.
I mean, if I were Catholic and someone had a world where Catholicism was the main religion ... I'd be a little hesitant. It would hit too close to home. That may not be true for everyone.
Last but not least, we don't really base D&D on medieval Europe, it's just the base concepts and language that we steal. D&D is a garbage can of myth, legend and cheap plastic toys used a miniatures.