I haven't watched the movie, but there are some possible explanations for the discrepancy you mention: 1. Perhaps all the other civilizations they've met so fa

I haven't watched the movie, but there are some possible explanations for the discrepancy you mention: 1. Perhaps all the other civilizations they've met so far communicate in a certain way (e.g. telepathy), and humans are the first intelligent species they've encountered that has the pesky requirement of audiovisual clues. They're just as busy trying to figure out how to talk to us as we are, but they're much more patient. 2. Perhaps their preferred method of space exploration and conquest is to wait until the indigenous species figures out how to communicate with them. Only then will they accept the indigenous species as worthy of participating in the Galactic Republic or whatever. If they wait a certain amount of time and we still can't talk to them, they'll either abandon us, exterminate us, or try again in another million years. 3. Perhaps their ship is in distress and stranded on Earth, with their Universal Translator broken and the aliens all cooped up in one room that still contains their atmosphere.