Humans are pretty bad at imagining forms of life different from our own. We’ve only encountered one habitable planet, on which there is only one species regarded as intelligent; in our vanity, we tend to assume it’ll look the same elsewhere.

We’ve never seen “sentient” microbial life and probably wouldn’t recognize it as such if we had, so who knows? Science fiction has imagined lots of things, but science fact unfortunately has a sample size of 1.