Hitchhiker's Guide - We're just not very interesting. The aliens exist, have noticed the Earth, yet don't care, because we don't really do much from a galactic perspective. Mostly Harmless.
Star Trek - The aliens exist, yet have some external reason to not interact, like a diplomatic treaty.
Star Wars - The aliens exist, there's just too many other aliens to deal with already (Republic). The situation external to Earth is some kind of massive bureaucracy that makes doing much other than dealing with the bureaucracy nearly unmanageable.
Culture Novels - The aliens exist, yet don't interact because we're the equivalent of barbarous housepets from their perspective.
Forever War - The aliens exist, may have noticed, they're just too busy fighting some really long term war with each other to care.
Stargate - The aliens exist, just view the Earth as a non-threatening farm to be harvested eventually.
The Algebraist - Aliens exist, yet the most common form is just really weird from a human perspective (like gaseous objects in cloud giants). Flesh bipeds are not all that interesting, or perhaps discounted because "intelligence only exists in these forms."
Dragon Ball Z or Warhammer 40k - Aliens exist, yet the Earth is boring. It's mostly calm bickering a lot of the time. Where are the enormous battles and massive weapons? No titanic fights that lay waste to the surface?