Because buying a cheap white t-shirt for 9,99$ ten times because it messes up everytime you wash it=100$, while buying a good quality white tee at Esprit or American Apparel is about 50$, and the chances of messing up/shrinking/turning grayish or yellow are way smaller.
Be on a budget, but some items require a higher investment. Like a white tee, or some makeup, or that chic little black dress you can accessorize a thousand ways. Even the good quality stuff is on sale sometimes!! Trust me!