So not only in inference but also in many other mental processes, the causal relation involves necessity.
But what about causation in physical nature? This is the field in wich most cases of causation are considered to occur.
The only relation between physical cause and effect is mere regular conjunction. You can't show that the relation of a hammer to a nail it hits is the same as the relation between statements in the process of inference and other mental processes.
"Why is any physical event followed by any other?
has not been answered. But failure in specific cases does not show that causality is merely regular sequence. Regular sequence is not enough. There must be some intrinsic connection between cause and effect. And this intrinsic connection includes logical necessity.
Mental processes supply genuine cases of causality, and any situation in which the causal relation involves necessity is enough to invalidate an explanation which denies that causality involves necessity.
But we are dealing with causality in physical nature. And even there, the theory is disproved by several criticisms.