It can’t be explained directly, by itself. They’re part of a bigger picture.
Straight men are attracted to women on the whole - legs, arms, waist, face, chest, back, it’s all good. Whatever force it is that makes us attracted to women, it’s not something that we chose or decided. We just do.
Since boobs are a sign of femininity, they’re attractive to us on exactly that level! They fit into our idea of what women look like.
There are great episodes in the first two seasons. You have to search for them though. (Please don’t miss Measure of a Man)