The Enterprise arrived out of no where in Q-Who?. They chased it, and then ZOOM. The bastard just left them for dust.
The Borg ignore inferior embarrassing species. To get their hands on Omega, the first bread crumb they assimilated was a low tech world that had a campfire story about "something way cool".
They might have thought "Q" was a human built stardrive in the beginning, a delicious piece of technology, but quickly they would have realized that the assimilation Humanity was merely a necessary stepping stone to the eventual assimilation of the Q Continuum.