Well, because most of them are literally vagina sculpted, but vibrators can be and often are penis shaped and molded. Dildos, doubly so. Furthermore, penis toys are often shaped like whole women, butts, labia, the whole nine. Now, given that it's an object being used for sexual gratification, I don't really see anything wrong with it, but there is an argument that could easily be made for how so many of them look very humanoid, where as many vibrators basically don't.