The middle indeed provides strength, when it is intact. It prevents the top and bottom from additional deflection under load as it takes on part of that stress. That is why there are very strict rules about when and how you can cut a penetration through a load-bearing joist (which, by definition, they all are).
Additionally, metal poles are a cylinder and take an axial compressive load, for which a cylinder is a very good shape; if you tried to support a floor with a wooden cylinder as a joist and the load was lateral to its length, it would likely buckle - its whole shape is hollow so it has the same problems as a joist with the middle cut out.
Every part of what you said is wrong.
The middle indeed provides strength, when it is intact. It prevents the top and bottom from additional deflection under load as it takes on part of that stress. That is why there are very strict rules about when and how you can cut a penetration through a load-bearing joist (which, by definition, they all are).
Additionally, metal poles are a cylinder and take an axial compressive load, for which a cylinder is a very good shape; if you tried to support a floor with a wooden cylinder as a joist and the load was lateral to its length, it would likely buckle - its whole shape is hollow so it has the same problems as a joist with the middle cut out.
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I’m really proud of how confident you are.