To go fully passive while keeping the same amount of performance you need to use much more material, so in this case copper and/or aluminium. Moving that air around with a relatively cheap fan works well enough and allows for a smaller build.

This is also why it's generally recommended to go with a classic air cooler that just has the fan speed set to the lowest level over passive air coolers in desktop PC-s. It's still quiet, but you get the cooling performance while using much fewer resources.