The reason people get scared of heights in buildings, on cliffs or ladders is because of the perspective of the structure disappearing beneath them - look down and you see the building width dwindling; you can feel your height and so on.
In an aircraft, there's a disconnect between you and the ground, so your hind brain can't say "ooooh, that's a long way down". There can obviously still be the fear of being at height ("what if the wing falls off?"), but not necessarily that gut-squirming, visceral, "this is plain wrong" fear of heights.
In a fixed-wing, there's not a huge amount of view outside, yup there are windows but there's also a lot of metal, helping protect our monkey-brain from the feeling that we'll will fall out of the tree. In helicopters, we're generally blessed with stupendous views out of acres of perspex, but as we climb this starts working against us - the acreage in view expands enormously so the seat/floor under us appears to shrink, until we feel we're precariously balanced and our inner chimp starts shrieking at us that the branch we're sitting on is really, really, really thin and likely to break at any moment.