It only takes one "Dalek" species in the galaxy
That goes out and exterminates every civilization they become aware of as a potential future threat. Even if 99.999% of species are curious and friendly (and I'm not so sure we'd qualify) the one aggressor would kill them off and dominate the galaxy unless an older friendly species saw them do that and decided to kill them off for the good of the rest.
We're probably better off looking rather than knocking on doors until we know a LOT more about what's out there. The odds don't favor the door knockers.