The fact that they're hidden also makes them more sensitive and arousing for the women. In some countries where the climate is warm and women don't cover their breasts the skin on the breasts is very different--a lot coarser, a lot less sensitive, like the hands or arms. This is especially true if the women do a lot of physical labour, as they tend to in hunting and gathering societies or some forms of agriculture (such as slash and burn). On top of that these societies usually aren't characterized by subscribing to any of the monotheistic religions that have put large emphasis on feminine modesty of the breast region, so they're less of a sexualized area based on that.
Or so they say. I really need to stop going into these kinds of paragraphs.