....I can confirm that spending time on European beaches and watching topless women from a very early age desensitized me. I know colleagues in continental Europe feel the same. I also know guys here will find it hard to believe, but it is simply the truth.
That would seem to answer your own question, "why and what is the US men's fascination with breasts" -- American puritan attitudes stifle desensitization.
Much has been speculated & written about the reasons, e.g.:
scholarly: men's fascination with breasts - Google Search
Tyro