Why DO men have nipples?

Because women are better!

That's my explanation. The explanation offered by the authors of the book Why DO Men Have Nipples (coming out Tuesday) is this:

While only females have mammary glands, we all startout in a similar way in the embryo. The embryo follows a female template until about six weeks, when the male sex chromosome kicks in. Men, however, have already developed nipples by that time. link

Ha! I think either way, you can conclude that women are better.