Because, they're vanilla. We're the basic choice. If you go to an Ice Cream parlor with a thousand flavors, and choose Vanilla. It's not a bad choice, just plain and a bit boring. That's why I don't want Humans. We're Vanilla in other works. We're pretty much in every work, because we're the only real sapient race on this planet. It makes sense, but it's just a bit boring.
I'm not saying it's a bad choice, just it's the only thing I do not want. I don't want to choose vanilla when we go into that ice cream parlor.