It does use Stellar directly. The real question is why use Stripe at all, and the reason being because for now Stellar doesn't have enough connected Federation servers that can translate coins directly to native fiat currencies. Or in other words I can't get the Stellar currencies out of Stellar and into your bank accounts without Stripe. This is hopefully only temporary though. Once more banks become federation endpoints and are issuing native fiat currencies on the platform the need for Stripe will go away.