Breasts are sexuals organs too, another tool of the reproductive process and there are not an organ just hanged out there for all the world to see. But as they are not your organs, as a male, you cannot never see and feel this way.
With a minimum of anthropological knowledge you know that throughout history women have been treated as inferior ( there are books where the stupid and ineffective nature of Woman by great " philosophers " is explained ) . The woman can not think , do not have enough intelligence to lead anything , women marry and only serves to give the man male offspring . Women do not vote, no opinion , just shut up and obey. And from this there are still countries where people live on these criteria, in the West a century ago it barely.
Under such a being made only for reproduction has only one way to attract a man, magnifying its most visible sexual organs : their breasts and buttocks, as animals do when they are in zeal. But in a fictional perpetual zeal.
Fashion in centuries was engaged in bulk with the rear yards of fabric , necklines became more generous. Men have never had to show off their sexual organs in order to survive , decide or get a wife . That's why they have remained there, well sacred and hidden from view of strangers.