There is nothing inherent in the process of evolution by natural selection that necessarily leads up to a buildup of complexity over time. But we see trends like an increase in body size, an increase in number of different cell types, an increase in number of protein-coding genes and in increase in total genome size.
Sometimes it is worth not considering the constraints of the world as they are now. Sometimes, it is worth thinking about what you would do as a scientist, if you had super powers, and could test any hypothesis you wanted, in any way you wanted.
Word is in from the streets of the microbe world, and the message is clear: “You multi-cells are always on about ‘niceness’ and altruism: well, we have those here, in abundance. And we don’t have to schlep around those heavy brains and wiry nervous systems of yours either.”