First, when an agent or a ghostwriter also publishes books they represent or write, the general concensus is that this constitutes a conflict of interest. The normal process of selling a book to a publisher is bypassed and quality control is compromised. Such agents or ghostwriters are naturally biased toward their own clients. Second, this kind of arrangement always entails paying the agent or ghostwriter (additional) money, sometimes disguised as a promotion fee, representation fee, etc. Essentially, it is vanity publishing, and such books do not make it into most regular mall bookstores. These arrangements are generally frowned upon by respected agents and major publishers in the literary marketplace.
