1. Most of the worlds evolved more to fantasy-like societies, like special-ability oriented, earthbound. For what we may know, special biological abilities akin to magic are norm, and tech is a deviation. (Total speculation).
2. Societies have space travel, but are very introverted in general and live inside mental macrocosm, rather than (space) macrocosm. (Read: "The Pod and the Barrier", by Theodore Sturgeon for what I'm referencing here)
3. There exists force that is extremerly dominant and prevents any free interstellar travel. May be active, may be passive relics, may be programmed bots/nanobots. (I think this one is highly likely, because it is extremely probable that some societies have say, at least ~10 million year technological headstart over others, and that is, paradigm-changing (pardon the word). We may not even understand such technology, and it unavoidably has, perfect cloaking, because at this point it would be trivial. We may already have invisible aliens among us, and we'd be none the wiser. In this case advertising our presence by use of nuclear weapons / other stupid activities (EM footprint, may be easily identifiable / detectable) may be a very, very bad idea, in case they are hostile / exploiter type.
4. Luck, which I think is totally unlikely.
5. They may already have left footprint (may be likely ?), if you think it's not the case, then explain the artifacts on Earth (pyramids all over the world aligned in certain directions (would require 1 world civilization which organizes building of them all to certain precise measurements, for all we know, they may not have been built by savage natives (pharaohs), but rather claimed by them by scribbling their writ on them), Moon being very peculiar distance from Earth and having very peculiar measurements, various reported phenomena found in coal mines reportedly hundreds of millions of years old (hard to prove, but interesting nonetheless)). DO NOT DISCOUNT WHAT IM SAYING HERE, IT IS NOT ALL CLEAR CUT AND IS NOT ALL"WOO CONSPIRACY". There are too many outliers found for them all to be natural deviations of a single civ (ours).
After all the considerations, same things apply even if light speed barrier is unbroken, because with high automation, you need only max 70K-100K years at lightspeed to totally map the galaxy, so if any civ existed in our galaxy, it is a certainty in statistical sense that it visited Earth either as bots, or living creatures.
All of the above is wild speculation taken with large doses of salt or soy sauce, your preference.