Because these materials are chemically unstable. PVC is a very hard plastic, and in order to soften it for use in a sex toy, plasticizers like phthalates are used. These are not chemically bonded with the toy, so the smell is the slow process of it breaking down. It is called off-gassing, and it is an indicator that there is a problem. I recommend purchasing silicone (the real stuff: Tantus, Fun Factory, Vixen Creation, Happy Valley etc). Or another inert product like stone, wood, glass, hard plastic (ABS), acrylic, lucite etc. They won't leach chemicals like Jelly and PVC (and others) can.