Many have tried to make humans co-exist with eachother, yet we still stab eachother in the back, if we cant live in peace with eachother, then another species is out of the question.
And in ME you can not coexist, by you know, killing of the coulncil, putting humans on top and helping out TIM.
...You missed the whole "have six with aliens" part, right?
Besides, backstabbing gets old when the knife in question is a railgun that can fire a twenty-pound round and the impact will make Hiroshima look like a slap on the wrist (say it with me, guys:
Sir Isaac Newton is the deadliest son of a [censored] in space!). If you've noticed, MAD kinda stopped most of the less-than-petty backstabbing, or at least major, catastrophic results don't occur due to backstabbing. (let's not go much further into that). Any other space-faring races would have heavy weapons, so we couldn't really backstab anything...
Not to mention that there's no pragmatic reason for letting most of the things in zoos live, but as humans we enjoy diversity and adventure. We try to preserve some of that magic, even if it's for selfish reasons, 'cause we like it. Hell, we even protect animals like alligators and crocodiles, despite the fact that as predators they don't take out massive numbers of species (don't quote me on this, they aren't the main predators in anywhere that I know of...) but can be really dangerous in urban places (quote me on this, I've seen it).