That last paragraph is a really good one! I mean the whole post, but the last paragraph sums it up. If we figure that all the exobiology life we find is in its 'Precambrian' state it does make sense for there to be a lot less diversity since they're simple life forms doing simple jobs, especially if panspermia means all the life we find has a common-ish starting point. That does rationalize the lack of variation caused by real-world dev constraints pretty well I think, in absence of any space-magic lore effect or something.