Because most of the time this fails. It depends on the terms of your agreement. Some attempts to rent to own result in the tenants loosing money. Other attempts at renting to own result in the final terms being so unappealing that the tenant/ purchaser will no longer want the home.
Renting to own is usually a bad idea because what is the market going to be like next year? Real estate contracts are complex because they need to be. Will the seller resist selling and be difficult to deal with if the value of his home goes up over the term of the lease?
Will the tenant not want to purchase if the value of the home goes way down during the term of the lease? I could go on and on covering reasons this is difficult. Thank you for asking about the main reason to avoid renting to own.