As far as I know, It's not known whether this is a causal relationship. It's possible that art that uses the golden ratio tends to be viewed as beautiful because of some magical aesthetic property of the GR, and that artists create art using this ratio intuitively.
But it's also possible that artists (and especially designers) tend to use it a lot just because they've heard that it's beautiful, when really it's nothing special. In this case, it would just tend to crop up in beautiful works because a) it crops up in all kinds of works anyway (base rate) and b) there's a whole lot of looking for it in hindsight (especially in works acknowledged as beautiful, while nobody goes looking for it in ugly works).
there is also an habit factor: after a few thousand years of using the golden ratio in objects we may have an unconscious knowledge of those proportions and find them beautiful simply because we have seen very often "beautiful" things that used such ratio.
But it's also possible that artists (and especially designers) tend to use it a lot just because they've heard that it's beautiful, when really it's nothing special. In this case, it would just tend to crop up in beautiful works because a) it crops up in all kinds of works anyway (base rate) and b) there's a whole lot of looking for it in hindsight (especially in works acknowledged as beautiful, while nobody goes looking for it in ugly works).
Some kind of A-B study would be nice. I found one (http://www.livescience.com/7389-sense-beauty-partly-innate-s...), but the methodology seems a bit lacking, I'd say it's far from conclusive.