Taking Trek as a model for what could be going on, assuming there are aliens and they are waiting for us to get to some point before revealing
themselves, then there's other factors besides being a peaceful world at work.
One of those is achieving a certain tech level - in Trek terms that means achieving Warp Drive capability. Before that level of tech achievement, the
Federation considers a world too backwards to be considered for inclusion. After that threshold they reveal themselves and make contact with the
world's authorities and offer friendship.
My point is, there could be multiple and varied factors as to why aliens are reluctant to reveal themselves so far. Tech level,
philosophical/spiritual enlightenment, world peace...these could be things. Or maybe the Galactic Empire prefers its members to have very strong
authoritarian ideals, and only opens up to worlds ruled by evil dictators...who really knows.
Another space opera that offers a compelling explanation of what could be going on Out There is David Brin's "Uplift" novels: Sundiver, Startide
Rising, The Uplift War, etc. In that universe, Earth is a forgotten backwater that only is even noticed by Inter-Galactic Civilization at large when
we build our first interstellar space craft. The politics of that civilization are very complex and involve billions of years of breeding and genetic
manipulation of lesser pre-sentient species into sentient, spacefaring species. Since the fabled Progenitors, the first species to achieve sentience
on their own, nearly all other species in the history of the Five Galaxies were raised up, or "uplifted" to sentience by another, prior race.
Humans seem to be an exception, we are a "wolfling" species who may have achieved sentience on our own. Some theorize that another species came a
few hundred thousand years ago and began our path to sentience, in secret (and thus illegally), and then abandoned the project before finished and we
evolved the rest of the way on our own.
By the time the books start, humans have themselves uplifted chimpanzees to sentience (there's spacefaring chimps with PhD's in the series
and we've begun the process for dolphins, too. The second book involves a mostly dolphin-crewed ship, with some humans and one chimp, who crash-land
on a world and discover a new species that is ripe for uplift - so by that point, only a few hundred years after humans (with their chimp and dolphin
clients/allies) are getting out into the galaxies, and starting to find alien species to bring to sentience as well.
Sci-fi has a lot of good theories to offer as to why aliens could be out there and not really making themselves known. Brin's books are some of the
best, I'd highly recommend them to anyone.