Well, because we are mammals and blessed with body hair, three middle ear bones, and the ability to nourish our young with milk that females produce in modified sweat glands called mammary glands. Although females have the mammary glands, we all start out in a similar way in the embryo. During development, the embryo follows a female template until about six weeks, when the male sex chromosome kicks in for a male embryo. The embryo then begins to develop all of its male characteristics. Men are thus left with nipples and also with some breast tissue. Men can even get breast cancer and there are some medical conditions that can cause male breasts to enlarge. Abnormal enlargement of the breasts in a male is known a gynecomastia. Gynecomastia can be caused by using anabolic steroids. So, if your favourite athlete suddenly develops man boobs and starts winning gold medals, you know the reason why.
Yep right on page 61 of "Why Do Men Have Nipples?...Hudreds of Questions You'd Only Ask a Doctor After Your Thrird Martini" It is the embryonic response discussed earlier in this thread. We all start off the same in early embryonic development and later the male sex hormones develop the male traits (about 6 weeks) but we are left with nipples and some breast tissue. This is why males can get breast cancer also. Get the book it is a pretty entertaining read with answers to questions such as: What causes an icecream headache? Can people in wheelchairs still have sex? Why do you laugh when you are tickled? What is snot? Is it dangerous to preform colonic irrigation on yourself? etc etc.
VERY EASY ANSWER-------- so that during sex our partner can hook up a pair of jumper cables to em and the other end to a car battery.....