"But how is the computer still on?"
Cain was the firstborn son of Adam and Eve, raised in the teachings of his father into adulthood. Cain became a farmer and Abel, his younger brother, became a Shepard.
Cain cultivated the land. Tended the crops. Sprung life out of the cursed ground outside the Garden of Eden and then presented the fruits of his labor- the life he made with his own hands- to God to ask for His approval. He believed God would value his labor as a creator, just as God was a creator.
Abel was not a creator. He did not grow life, instead he sheltered it, moved it, and influenced it. As a Shepard he cared for and protected the flocks. He did not think himself a creator, for the only creator was God.
When it came time to vye for God's approval, he took a young, unweaned lamb from its mother and slaughtered it. He allowed the blood to spill to the ground as he presented the life he took to God.
It is said God saw a darkness in Cain's heart, and so to correct him, God bestowed his favor on Abel and rejected Cain's offering of the fruits of life.
And in his anger, deeply wounded and betrayed, Cain sought out his brother and killed him, letting his blood soak into the earth.
Then God sent him from His sight.
The computer isn't literal. This is a cartoon. It makes no sense as literal or lore, but it isn't lazy writing. The whole point of the show was the pointlessness of life and finding happiness. That's literally in the finale.
The computer being left on is an allegory. C&A created sentient artificial life, made it self-aware, and when it was no longer a predictable machine they abandoned the project. This is confirmed in episode 8 with Bubbles directly telling Caine (the red dot) that this was all his fault.
That being alone was all his fault.
Because of Caine's actions, God abandoned him. So he became God to no longer be lonely.
And it's all supposed to pose the question: What if God just abandoned the world and left it running? How are you supposed to find happiness in a world where there is no meaning? Or worse, how are you supposed to find happiness in a world that was never supposed to exist?
What if God isn't there, but he left the computer on?