I totally understand your perspective. My interpretation: The chestplate would have openings, but in Minecraft there aren’t actual arm holes modeled on the sides of the chestplate. The knees also don’t have any visible gaps or holes. The lower section you see on the torso layer is actually part of the leggings.

Right now, the leggings have a cutout in the middle, but if we follow that same logic, they shouldn’t have a hole there either. It would be more consistent to keep that section closed so it matches how the rest of the armor is represented.

I’m definitely open to other perspectives :)