In createReducer, why 2 nested functions instead of just 1?

[ppo]

In createReducer, why 2 nested functions instead of just 1?

Why (initialState) => (reducerMap) => (state = initialState, action) =>
instead of (initialState, reducerMap) => (state = initialState, action) =>?
As it's anyway always used as createReducer(…)({…}), right?