Why do you think it hasn't happened yet?
Imagine a race that kills it's own without abandon. Over the smallest differences. Height. Skin tone. Eye colour. And even non-visible differences. Like basis of belief. Their culture. Etc. Things that can change in only a hundred years, or less! Look at the culture change in the USA in the last 50-60 years alone. A race that has been at war with itself for over 5000 years straight. That has industrialized even the killing of it's own people.
Now give that race a common enemy. A common thing to fight against. Especially one as so different as an alien race and culture (assuming they are that different). Not only would we rally against the physical differences. But also the cultural differences. Now imagine what we would be capable of?
Think any alien culture is dumb enough to open that door? As the story goes of what the japanese said after the bombed pearl harbour "I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve."
I almost bet that just outside the reach of our probes, our current ability to see or detect. Are beacons. All around our solar system, that loudly scream to any passing vessel. That our system is off limits. Not to be entered under any circumstance. Lest they "wake the sleeping giant".
