Honest question: why not use both? If spinning up cloud computing resources is the most cost-efficient way to do those computing tasks, why not have that be the

Honest question: why not use both? If spinning up cloud computing resources is the most cost-efficient way to do those computing tasks, why not have that be the route for doing one-off computing tasks while your own machines run the workload for recurring stuff? Is there overhead for not having 100% of your computing done on a cloud platform that I'm not aware of or something?