They are just the biggest asshole in the room.

So one day the different body parts were arguing over who should be in charge.

The eyes said they should be in charge, because they were the primary source of information about the world.

The stomach said it should be in charge because digestion was the source of energy.

The brain said it should be in charge because it was in charge of information processing and decision-making.

The rectum said nothing, just closed up shop.

Before long, the vision was blurry, the stomach was queasy, and the brain was foggy.

Assholes have been in charge ever since.