I mean, we don't have any other perspective to look through. There is no other point of reference to draw from. We try to compensate by assuming that our system, planet, and species aren't particularly special and are probably about average in terms of supporting life.

We can't know or even begin to guess at what an alien civilization may do or think or how they evolved. Best we can do is assume it's probably somewhat similar to our experience. At least it's based on something factual. Anything else is really just wild speculation.

We pretty much have to assume aliens will be sort of similar to us because we haven't met any. Our experience is the only one we've got, so it's the most reasonable baseline we have. We know that aliens will probably be wildly different from us, but it's so unknowable as to be moot. Do we base our assumptions on Heinlein's writing? Asimov? Douglas Adams? Anything other than what we know from our own experience is just fanciful fiction.

But also you're not supposed to take as read the Fermi paradox, Kardashev scale, or any other ways of thinking about aliens. It's implied that they won't be anything like us. You're not supposed to take it as a literal statement that alien species will be hairless bipeds with a warlike society who think and look like us. You're supposed to follow the assumptions that statistically, we're probably not special as a species and probably any aliens we meet will have evolved along similar lines and probably will be relatable to us. Implicitly we understand that this likely is not true. We just don't know and there aren't really any options that are more reasonable or reliable than basing assumptions on the one and only planet we know that has intelligent life.

This is too pessimistic. We can absolutely "begin to guess" at what an alien civilization might look like or how they might have evolved. Reasonable possibilities are very heavily constrained by information theory, the laws of thermodynamics, the very finite number of elements that make up the universe, chemistry, particle physics, etc. You can appeal to fantasy by dreaming up multi-dimensional energy beings, and of course such things have some tiny but nonzero probability of existing. But you can do the same thing regarding what we might find if we look at some random pond water under a microscope. Maybe we'll find crazy silicon-based life forms, or some phylum we never knew existed, or a microscopic Tyrannosaurus Rex! Maybe. But it's perfectly reasonable to ascribe an extremely small probability to such things based on everything we already know about the universe, and it's reasonable for the same reason to ascribe a very small probability to the existence multi-dimensional energy beings that would completely defy our understanding. For any given alien life form we might find, there's a very high probability (I give it at least 90%) that it would be based on chemical pathways that would make us go "oh, neat", but not go "THIS CHANGES EVERYTHING!"