If I specified sheet aluminum for Type II Class 3 chemical conversion per MIL-DTL-5541, any materials QC professional would be hard pressed to visually determine that anything was actually done to the material.
To be sure, no, MIL-DTL-5541 isn't an uncommon spec constrained to military applications.
Polished aluminum is not low maintenance by any stretch.
And I think the... it seems they're called Krueger flaps? Those are often painted aren't they? And those I'd think you'd definitely want to de-ice.
Did you mean it’s not aluminum? Or a different grade?
Yes, well, they may either be materials that have other surface textures/coatings/treatments/finishes that aren't conducive to polishing, or they may be other metals, or non-metallic composites (can't put aluminum over a radar, for example), and so on.
TL;DR: The painted surfaces are generally instances in which polished aluminum won't really work, for a variety of reasons.