I think it's because of the fact that white girls look like manly macho tough guys especially when it comes to their faces. Even the white girls that Westerners (such as Americans) perceive as the most beautiful look like intimidating aggressive hypermasculine tough guys to us Asian guys because it's the truth.
Asian women on the other hand, physically look attractive, beautiful, and feminine especially when it comes to their faces. Seriously, most Asian guys I know consider Asian women to look like angels, princesses, and sacred maidens while they consider white girls to look like warlike ancient Celtic warrior males that battled the ancient Roman empire. Those Asian guys are right.
So what do you think of this perception of white girls from most Asian guys I know? For me, I believe in their perception.