Why do you think Greek city state invested substantial resources in temple building?

Greek City-States invested in temple-building because these states were temple cities. Temple cities represent the merger of the religious and statecraft functions in the structures and monuments in the city-state. Moreover, during classical antiquity, human beings believed in the active engagement of the gods in human affairs. Temple building was viewed as an essential state function which was necessary to ensure good luck, maintain the priestly class and honor the gods for whom many annual festivals were named.