Possibly for the same reason that women have orgasms:
Embryonic development is typically a package deal, with various traits dependent on preceding stages. Since male and female embryos undergo a common developmental pathway, it would be a major fitness burden to interfere and redesign an unnecessary independent pathway that led to nipple-less men. After all, if nipples don’t do any good, at least they don’t do any harm, so presumably they are simply along for the ride. If they caused trouble, perhaps if they frequently became cancerous or if they required a lot of metabolic energy to create and maintain, they doubtless would have been selected against, but they ain’t broke—merely irrelevant—so evolution hasn’t fixed them. The same applies, or so it is claimed, to female orgasm.
This idea isn’t quite as ridiculous as it seems. Just as male orgasm and ejaculation occurs when the penis is suitably stimulated, female orgasm is intimately linked to stimulation of the clitoris, and the penis and clitoris both derive from the same undifferentiated embryonic tissue, called the genital ridge. Both penis and clitoris are therefore richly endowed with nerves and with parallel brain mechanisms that respond—orgasmically—to enough of the right input from them. It’s just that in one case (men) that response is adaptive, whereas in the other (women) it is essentially an evolutionary hitchhiker.
The Dish 