I would say, "optimization rewards offense over defense." Optimization creates a game where PCs are required to one-shot opponents to stay relevant. In this kind of game, initiative and offense is 100%, and defense is irreverent. The GM can easily optimize monsters for similar offensive capability, however, frequently killing characters through rocket-tag is generally considered not fun by players, and is typically very disruptive to a campaign. The default for Pathfinder is that the GM is supposed to lose almost every battle, and optimization requires that lose to be immediate.
Since PCs and monsters have access to most of the same stuff, Pathfinder rules balance offense and defense fairly well, however it is fairly easy to disrupt this balance (within the rules) and encounter design and pacing can have a huge effect. Sadly, even many published adventures feature many single monster encounters or other recipes for disruption.