The Roman Empire started much later than the Greek polis.  Therefore, the polis was the form of government that came about first.

The Roman Empire was not even the first form of government within Rome itself.  Instead, it followed both a monarchy and a republic.  Rome did not become an empire until the first century BCE.

By contrast, the polis arose in Greece centuries before.  There is evidence of the existence of this political form in Greece as far back as the 7th century BCE.

This is not to say that no empire ever existed at all in the world before the polis arose.  But the Roman Empire did not arise until centuries after the polis.