I don't understand why you see it that way... Whether using an arena allocator or smart pointer, in both cases you need to allocate the individual object (p = arena_allocate(arena, ...), or p = std::make_unique<T>()). In the case of a smart pointer that is all you have to do - deallocation will be automatic. In the case of the arena allocator you will also need to deallocate the entire arena at the appropriate time.

How are you conceptualizing this that the arena allocator is simpler?

How are you conceptualizing smart pointers as "obscuring" ownership, when the entire point of them is to make ownership explicit! The smart pointer IS the owner!