We all know where nipples are located on the human body. It doesn't need to be grossly emphasized. We will safely assume that wherever the boob starts to slope inwards and downwards again is where the nipple should be, and we will extrapolate and apply that information to our perception of where that shirt will hang on our own bodies. Furthermore, being able to see that the nipple is clearly visible through the fabric is not necessarily a way to entice a woman to buy an article of clothing. Not all women care to be that cold, or invest in silicone nipple covers, or thick bras.
(I've been noticing this more and more as Christmas approaches. Who thinks a woman wants to wear a wee scrap of gauze and shimmeryness to a party when it's 20° below zero and her coat is doing nothing to keep the wind out?)
Similarly, guys, don't be intimidated/disappointed that the underwear mannequin has a huge package/flat frontpiece. When is everyone going to realize that mannequins are not an accurate reflection of reality, hello!!!