There's actually a couple reasons why I don't buy kits.
Number one, my fiancé and I are on such a shoestring budget that I just can't afford to get a kit. When I decide to do a quilt, it will take me a couple of months just to get the fabric together, then another month to buy batting, then yet another to buy the backing. Since I bought a bolt of Muslin, that should change things up a bit, but not by much.
Number two, when I do find a quilt kit that I find interest in, nine times out of ten, I'm not thrilled with the color choices and I pass it by. I'd much rather work in EQ, pick my own colors and fabrics and make a quilt that way. If I feel that my brain needs a break from quilting, I work up something easy like a nine-patch or do up HST's. If I need a bigger break, I break out either my crochet hooks or knitting needles and play in some yarn for a bit.