This is economizing with a vengeance, and sometimes it doesn't makes sense from a more far-reaching point of view---but the weakness of natural selection is that it has zero foresight. It works just like corporate decision making: a slight economic advantage here-and-now, at the cost of a huge prospective disadvantage, wins the evolutionary race every time. That's how we lost the capacity to make vitamin C---at some point, when the ancestral monkey was a fruit-eater, there was a slight energetic advantage to no longer retaining the pathway. Future generations received a major handicap, but the future is invisible to the natural selection process. There are countless examples like this: the pathetic thermal inflexibility of most marine species, diabetes (which confers a very slight advantage in times of famine). . . Apparently even a minuscule energetic saving is a very powerful evolutionary selection factor.