Sacrosanct
Legend
Because it is the nature of creatures to exploit/hunt/use those less than themselves. Not limited to humans. The right question, is "Why wouldn't they?" I mean, if they are going to spend the energy to visit us in the first place (rather than completely ignore our insignificance), there needs to be a reason for that. And when a creature is motivated to seek out another creature less than itself, how often does that work for the benefit of lesser creature? Especially when that lesser creature is violent, unpredictable, and self-destructive like humans are?Why? If they are truly alien, we can't know what they might do. And if they are psychologically like us, they wouldn't necessarily think of us as "lesser" beings.
If aliens can travel between stars or dimensions, there is nothing our little rock can offer them that they can't get in more abundance elsewhere -- except culture. Except us. They aren't going to strip mine our planet or enslave us. There's no motivation to do so for an advanced species. Their only possible interest in us is what we are -- moderately smart monkeys that likely represent their own ancient history in some way.