War in a game should not be.
War in a galactic scale does not exist. (As far as we know at least.)
So first of, it's a realistic experience. Just because you want to push a button, win the game and become god of the universe doesn't mean that you should. If you want that, go play rigged slots or something.
Stellaris and PDX Games in general revolve around playing the long game: wars are long, tedious and rely on extensive plans. So does planet Management. It's not about Instant gratification.
Secondly, you are using a proof of if ignorance fallacy to justify your own dissatisfaction.
Let's put your logic into action, just so I can show you how faulty it is.
By your logic, because a supernova hasn't hit earth, we cannot prove that it would cause the earth to cease to exist, or at least be heavily damaged. We also can't prove that it won't turn all the sweet potatoes, a tuber I loathe, into cupcakes.
And thus, since I would greatly prefer the latter, I will demand that a supernova simulation another person developed must have all supernovas turn sweet potatoes into cupcakes. People will think I'm crazy and that this adds almost zero value to the simulation, but that's your logic.
Now, some basic facts about space you might not know. First off, space is VAST. I mean really big. It's bigger than your backyard! Its bigger than the sun! I know, that might be hard for you to understand, but space is a really big place. And, going between one point to another point takes time. So, if you think about it REALLY HARD, you can come to the conclusion that it would take a lot of time to go from point A to point B on galactic terms.
Am I going to fast for you here? Am I using to many big words? If not, I'll continue, because it's going to get a bit more complicated.
So, when you fight a battle, you loose ships, unless you win a very one sided battle. It is very smart to stay behind and defend the system you just captured while recuperating, which takes time. It takes time for reinforcements and dedicated defense fleets to arrive, it takes time to repair, and it takes time do invade and fight battles and even to just move between star systems.
Combine that with the fact that you have to continue to do this on repeate to capture more planets and systems, and defeat enemy fleets, and you can come to the conclusion that all this takes a lot of time.
Now, if you want to get your alien friend to refute basic astronomy and basic war principals, be my guest. If not, please refrain from inserting your personal dissatisfaction into a game with a war system that works quite well, beyond the sloppy war exhaustion mechanic.