It just looks that’s what worked in evolution—to have independent organisms, each carrying its own brain. And the brain happens to have the richest information processing and integration, compared to information processing between the brains.
I don’t know what would be necessary to have a more “joined” existence. Mushrooms seem to be able to form bigger structures, but they didn’t have an environment complex enough to require the evolution of brains.
It just looks that’s what worked in evolution—to have independent organisms, each carrying its own brain. And the brain happens to have the richest information processing and integration, compared to information processing between the brains.
I don’t know what would be necessary to have a more “joined” existence. Mushrooms seem to be able to form bigger structures, but they didn’t have an environment complex enough to require the evolution of brains.