Actually it looks like this might not necessarily fully explain why.http://www.mit.edu/~krugman/evolute.html
The answer is surely the ever-present need to simplify, to make models that are comprehensible. The fact is that maximization and equilibrium are astonishingly powerful ways to cut through what might otherwise be forbidding complexity - and evolutionary theorists have, entirely correctly, been willing to adopt the useful fiction that individuals are at their maxima and that the system is in equilibrium.
Let me give you an example. William Hamilton's wonderfully named paper "Geometry for the Selfish Herd" imagines a group of frogs sitting at the edge of a circular pond, from which a snake may emerge - and he supposes that the snake will grab and eat the nearest frog. Where will the frogs sit? To compress his argument, Hamilton points out that if there are two groups of frogs around the pool, each group has an equal chance of being targeted, and so does each frog within each group - which means that the chance of being eaten is less if you are a frog in the larger group. Thus if you are a frog trying to maximize your choice of survival, you will want to be part of the larger group; and the equilibrium must involve clumping of all the frogs as close together as possible.
Notice what is missing from this analysis. Hamilton does not talk about the evolutionary dynamics by which frogs might acquire a sit-with-the-other-frogs instinct; he does not take us through the intermediate steps along the evolutionary path in which frogs had not yet completely "realized" that they should stay with the herd. Why not? Because to do so would involve him in enormous complications that are basically irrelevant to his point, whereas - ahem - leapfrogging straight over these difficulties to look at the equilibrium in which all frogs maximize their chances given what the other frogs do is a very parsimonious, sharp-edged way of gaining insight.