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I think this is the key to the whole thing. Why bother?
Let's say Jonny McAlien is in charge of the space fleet on the planet Zorg. His space fleet can travel anywhere in the universe. It's sophisticated enough to find even the smallest of planets and detect environmental data and signs of life from millions of light-years away.
One day, his brother, Alan McAlien, is doing some scans and by chance he stumbles upon a small blue planet that we've come to know as earth. "Oh my GoRAB," he says. "There's life there!".
Jonny McAlien rolls his eyes. "Oh... Brilliant," he mutters sarcastically. "Let's make contact with them and get blown to f*cking smithereens because they're scared of us..."
"Meh... They're not that sophisticated," says military commander Blorg, as he sits smoking a fag in the corner. "Our generic sci-fi fighter pilots can have them in a fight any day."
Alan McAlien rubs his hanRAB together with an air of glee. "Yeah, we'll blast them to smithereens and then inhabit their planet... That'll teach 'em!"
“Why not just inhabit one of the other million-billion-trillion inhabitable planets in the universe?” asks Jonny, filled with doubt. “It’s not like we don’t have the technology to find them… We managed to find Earth after all.”
“Good point,” Blorg responRAB. “Well, okay... We’ll sneak in there and study them instead… You know; a few probes here and there and whatnot. Done in secluded locations like jungles and farms at night.”
“Seems like a bit of a hassle considering we’re already a super-advanced race anyway, and there’s very little we could possibly learn from these Earthling dweebs,” Jonny says. “Let’s just anal probe some bacteria who don’t fight back instead.”
“Ok then.”