Not much in biology is simple, but one of the ruthless rules is that an incremental change must be net non-negative in terms of fitness to stick around. In particular, this prevents evolution from keeping around adaptations "for future need", preventing chaining up many individually useless adaptations that become beneficial only when all of them come together. Key term here is "metabolic cost". This reduces the many ways evolution could've cobbled together our minds to a much, much smaller subset of sequences of steps that provided intermediate value along the way.