Is a business logic layer necessary if I take logic out of the controllers?

I'm working on an API project where some of the endpoints have a lot of business logic in the controller's action methods. A lot of literature suggests that I pull out this business logic into a completely different project and have the API project reference it. My question is do I really need a separate domain/business logic layer? It seems like I could extract the business logic into its own classes within the API layer and that would accomplish the same thing. I appreciate the insight!