The universe is old. If habitable planets are common, then it's functionally inevitable that intelligent life exists with a billion year head start on us.

Would they eschew expansionism? Maybe. Would they always eschew it 100% of the time? Definitely not. And this is a Darwinean process - the ones that decided to stay home are the ones you don't meet.

And they've had a billion years to change their minds. Consider that the likelihood is numerous billion-year-head-starts, one species would inevitably try it.