My parents are buried in an archdiocese of Newark cemetery. Our entire family has plots in that cemetery and over the years we’ve had to purchase headstones from a business outside the gates. If the archdiocese starts selling headstones, they can always compete with the archdiocese by meeting the price but offering better service and quality of workmanship. When the playing field is equal, the service can set you apart.
Well, meeting the price means actually being lower in price since the church doesn't have to deal with soul crushing taxes but the businesses do. Tough to match costs when the playing field is tilted that way.
Not to say they cannot compete, but it will have to be on different terms. Innovation, for example. Try "fun" monuments like cupcake shaped or something. Or create accurate effigies of the deceased. Come up with something that the church just won't offer and fill that niche.