This is one of the reasons I love space. It's one of the few things that truly makes my problems seem so small and insignificant and it reminds me that other people are still valuable even if I really don't like what they do or say. We're in this together and there's so much for us to learn and explore.

Short answer is that we don't assume that. Not sure if you've heard of the fermi paradox but the solutions proposed to that seem pretty relevant to what you're asking here.

Long answer is simply that we don't think that but that our discussions typically revolve around advanced civs because they pose the most interesting and relevant discussions to our world today. We only talk about more advanced civs because the types of aliens relevant to our own interests and security would probably be advanced. A hunter-gatherer civilization probably wouldn't be a threat, we wouldn't be able to communicate with them from long distance, and they wouldn't visit us here on earth so we just don't talk about them as much. Scientifically speaking, aliens could be anything, but we just don't know. Your discussion above is one of many possible conditions of extraterrestrial life.

Theoretically, you are right. We haven't proven that we aren't currently flying off into space right now, but based off of what we can observe and know, it's very likely that we are orbiting our galaxy (not necessarily SagA, but, rather, a sort of galactic barycenter). It's not a fact in the scientific/mathematical sense of the word, but the probability of any other explanation is so low that we can treat it as such and form our assumptions around it. That's how most high-level science and mathematics work though. Many things that we treat as facts or proofs are actually just theories and very educated guesses, but they work very well for us and help us innovate and learn about the world around us; sometimes we don't have to prove something for it to be useful to us.

Sometimes all it takes is finding a way to apply it to something you are passionate about. Math in a classroom is usually boring as hell. I took Calculus classes twice, hated it both times as I dropped out the first time and barely passed the second (first in high school and then in college). Now I'm doing it again in software development and it's making perfect sense and I'm really enjoying it.