because grapes are perfect and grapefruit sucks, so grapefruit was all like "yo grapes, help me out here no one likes me" and grape was like "yeah i know, i rule even when im fucked up im a raisin, i guess you can borrow my name to help you out, but heres the deal....when we are in a fruit salad together, you know how you make all the other fruits taste like you? yeah, well dont FUCK UP MY SHIT anymore and get your juices all on me and you can use my name" and grapefruit was like "aight"...so thats why its called a grapefruit, and thats why grapes dont get fucked up when you put them in fruit salad with grapefruit.