A random walk in the space of complexity can’t go below zero, so the only other direction is up. If going up is more difficult than going down in that space, then the general trend should be to retain simplicity, with some rare occasional offshoots towards greater complexity when and if that is adaptive. That is, in fact, what we see.
Most of life has remained as simple as it appears possible, manifesting essentially as internal genetic parasites like viruses and selfish DNA/RNA such as introns and other forms of transposons, while some few offshoots have increased in complexity.
It should be noted that viruses, which are so simple many biologists don’t even consider them a form of life, is also the most abundant semi-biological phenomenon for lack of a better term. The 2nd-most abundant biological phenomenon, while complex enough to be considered life, are bacteria, the simplest form of life known.
Complex life did evolve, but it is a bit of an anomaly in terms of the number of lineages that have evolved increased complexity. It is also noteworthy that most genetic complexity in terms of the amount of genetic material, is actually junk DNA, and there seems to be no obvious relationship between multicellular organismal complexity(whether measured by number of cell types, their methods or types of interaction, or spatial arrangements) and genome size.
To explain the relatively few lineages that became complex really doesn’t require any more than a random walk in complexity space. The organisms that became complex(plants, animals, other multicellular eukaryotes) also became spatially large and macroscopic, and adapted to basically any enviroment, so it feels like they are everywhere and the most dominant form of life just because we are them and can see them everywhere around us. But most life, in terms of the number of individuals and the number of lineages, and the amount of time it has persisted, viruses, selfish DNA, and prokaryotes, is and has remained relatively simple for it’s entire known history.
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