Because we have multiple different motivations, arising from different drives. We have the drive to be social, which can be good in making us inclined to help people, but bad in making us value social standing above other things. We can be hungry, which means we are more inclined to grasp at what we want even if it is unfair in other ways. We are fearful of instability, which makes us less likely to tolerate the freedoms of others. We have a goal to have and raise families, which can lead to competition and putting ourselves and loved ones above others who may be more deserving.
Is this not obvious?
What has this to do with deities?