I have heard of wild aros being caught with drop eye. I believe it, too. Think about it, it their "natural" habitat so different? If there is a lack of bugs and other food from the surface, will they not look for food elsewhere? If the river floods so that it is deeper?
Also, is it also possible that it is an old age thing? Like wrinkles in people? Certain things can make it worse or help.
Are all these not very possible. Kinda like diabetes in people. Healthy people can get it, they don't have to be fat and overeat.
The bottom line is we don't know exactly what causes drop eye, just what it is. We can fix it and even prevent it to some degree. Remember, poor water quality (especially high nitrate), poor nutrition/diets, tank depth, tank design, tank mates, and many other factors have all been identified as possible causes. People sometimes have fed an aro a very lean diet and put it in a shallow, wide tank with no tank mates and excellent water quality and sometimes drop eye eventually develops.
Until a large scale long term study is done on arowanas and drop eye, we can only do our best to prevent it. And remember, it does not hurt the fish. It is simply a new eye position. It does not even make it look bad in my opinion.