Earth is a very fertile planet for life. Really the ultimate "Goldilocks Planet." So why haven't aliens colonized the place in the past 65 million years or so? Maybe because they set it aside as a nature preserve. Their thinking is that the diversity of life here would allow them to harvest all kinds of pharmaceuticals while learning all kinds of solutions to environments ranging from extreme cold to extreme heat and everything in-between.
But then we humans evolve, and we start wiping out all kinds of other interesting forms of life...hmmm...