The aluminium chassis cannot be used for heat dissipation without risk of harming users. Which is why there is a "macbook air peformance mod" to add thermal-interface-material (instead of thermal insulation) to turn the chassis into a heatsink.

It's not a heatsink by default.

Air has no thermal connection to the chassis for the purpose of making it safe to have in contact with skin.

People have been modding theirs to make this contact, though. And been getting a significant performance boost out of it.