Plants on Earth play an incredible optimization game. Water is necessary not just for chemical metabolism, but because throwing it away is an essential part of the internal material transport systems of a plant. Evaporation in the leaves creates a relatively lower osmotic pressure in the higher levels of the plant. Between capillary action and the slight but insistent pull to replace evaporated water in the leaves, plants have a pumping mechanism with essentially nil energy input: minerals and nutrients gathered in the roots just ride on up with the tide.

Stars may be a similar story for efficient fusion. Sure, with sufficiently advanced technology, I can make inordinately denser energy sources with less boil-off. But do I want to? Maybe letting the brute physics of a gravity well do the job simply has unbeatable efficiency -- just like evaporation as one end of a pump for plants on Earth is clearly no comparison to any motorized pump for volume, but for cost-effect, nothing else comes close.