Mine is probably the most boring: There are many intelligent species in the universe. Faster-than-light travel, however, really is simply impossible, meaning that there cannot exist a truly interstellar civilization. So while some species have probably settled solar systems other than their own through generation ships, suspended animation, time dilation, or whatever, their range of expansion is limited. This means that encounters between species of different planets are rare. Humans will most likely never contact any intelligent alien species, at most one or two. We might, however, discover evidence of their existence through telescopes or something.
Any K2 civilization would be observable from earth, provided they've been K2 long enough for the light to have reached us.
Right, exactly. To me, the fact that we haven't seen any evidence of megastructures suggests that there cannot exist such a thing as a K2 civilization, or anything particularly close. We can see millions/billions (not sure on the #, see below) of stars, but apparently not one sign of a megastructure. This is a much broader sample size than with conventional SETI listening for radio messages. I'm confident that if even one megastructure has ever been built in the history of the universe, there would be at least one that we could see.
Or at least that's what I was going to say, but apparently only 10,000 stars are visible to astronomers, which is a fairly small sample size. So who knows.Wait, what? With telescopes and shit there's no way we can only see that few.
Oh wait, I misread the search results. 10,000 is the total number that can seen from (all points combined on the) Earth by the naked eye. I can't find a number for, say, the Hubble, but it seems to be much higher.