That is rather a sweeping generalisation. In actual fact some species grow on top of branches or even sides of trees, and the habit is exactly the same as other orchids - everything is the the right way up. But they are not all the same. I have seen one of the large white flowered species growing on the underside of horizontal branches, at a well- known nursery at Butterfield, in Malaysia ( opposite Penang Island, on the mainland) and was told that they grew them that way because that is exactly what happens in the wild with that species.
I cannot remember the name- at one time I thought all the large white ones were one and the same species, but now I know much better : amabile, Aphrodite, phillipinensis, stuartiana alba, and new ones distinguished all the time.....
That nursery visit was a long time ago , btw, probably when it was run by the grandfather of Michael who runs it now, so I am told.