Because Inter$-$AS routing protocols $($e$.$g$.$ BGP$)$ only run routing algorithms, only perform forwarding, and are NOT responsible for providing end$-$to$-$end connection$-$oriented services $($these devices don't even require an Application layer anyways, as end$-$devices do$),$ they have NO NEED for a reliable transport$-$oriented protocol as end$-$devices do$,$ nor there is ANY evidence of such.