Science Fiction

Certainly there's no evidence of other civilizations, but space and time are vast, other civilizations could have risen and fallen before humanity ever came to be and evidence of their existence lost to time. They could exist in places humans wouldn't look, possibly even in forms we wouldn't recognize, or maybe they even purposely avoid us, folding space and time around themselves, hiding from prescient vision, etc. Maybe they're even out there and humanity is aware of them and they just have nothing we want and they're not posing a threat so they're never really worth talking about in-universe, certainly wouldn't be the first time humans didn't give a shit about another civilization, not even the first time in the scale of the books like how the harkonens never game much serious consideration to the fremen.

That's of course all just baseless speculation based on basically nothing in the books, pretty much just the old "absence of evidence isn't evidence of absence" thing.

Also, depending on what we're willing to accept as a sentient species, what your interpretation of the butlerian jihad is, how you feel about Brian's books, etc. there's also the matter of thinking machines to consider. I'd personally call that a stretch, but we're here talking about a weird sci Fi book, so weird sci Fi stuff isn't off the table.