1
And it's an honest question, not a counter.
If the idea of danger weighs heavily -- and it probably does -- wouldn't *one* of those alien races have fired off a "berserker" network of self-replicating machines to sterilize the universe and protect itself?
1) After all, one of the other species is going to come and make contact with YOU :- )
2) You don't have to imagine EVERY species doing that. It only takes one species to nuke Hiroshima, as it were. And it would be very, very easy to program, and to decide on -- even if you regretted it later.
........
And wouldn't we (our own species, Man) therefore not be here?