> I often wonder how more biologists aren't believers

Many observations are unexplained, and not just in biology. The difference between believers and atheists is that atheists stop there: it is unexplained, at least for now, that's how things are. Believers will instead attribute it to god, or some other form of higher power. In the end, it just shifts the problem, at some point you will have to admit that some things just are and there is no explanation, for atheists, these are the things themselves, for believers, it is god who made the things.

That's why being a believer or being an atheist doesn't have much to do with being a biologist. It is just a philosophical view of how you deal with the unknown and the unknowable. The only thing is that the religious dogma should not get in the way of proper science. That life is so beautiful that god must be behind it (are we still talking about poop cubes?) shouldn't prevent biologists from searching for an evolutionary explanation.

Also, evolution is mostly random, not everything needs natural selection, sometimes, things happen for no good reason. Maybe a particularly prolific male in a particularly successful colony happened to have a square poop mutation or something.