I don't think the argument is that it "has to have happened another time". The argument is that if it is possible to happen once, it's likely to have happened m

I don't think the argument is that it "has to have happened another time". The argument is that if it is possible to happen once, it's likely to have happened more than once, the "existence heuristic." It proves only that it might be worth looking, unless perhaps further research indicates that we on earth are not, in fact, intelligent life, and then the likelihood of intelligent life elsewhere in the universe falls off dramatically.