Well, it’s because the more precise words don’t really provide more information. And honestly, we do this exact thing constantly in everyday life without thinking of it as confusing.
For example, “Usain Bolt is the fastest man alive”. Sure, that we know of. There might be some kid in Chile that’s .05s faster, but we’ll never know because his life circumstances don’t bring him to our attention. “The Bugatti Chiron is the fastest car anywhere”... except for that speed demon tearing up the tracks in orbit around Alpha Centauri. “Stephen Hawking is the smartest guy”, if it weren’t for that absolute genius herding goats in Siberia who didn’t get to go to school and has the local reputation as the weird kid arranging the livestock in novel patterns. “Tuna is the most delicious fish” because no one’s thought to make sashimi of that weird looking thing we dragged up from the Mariana Trench.
In all of those cases, there’s an unspoken “...that we know of”. Even if you believe that each of those cases is the fastest, smartest, or otherwise best, you have to concede that there may be another faster, smarter, or better elsewhere that we haven’t discovered yet.
So why pick on cosmologists? It’s literally impossible to declare anything to be “the most $X in the universe”, first because the universe has an infinite amount of stuff we’ll never be able to see, and second because we haven’t even remotely finished cataloging a zillionth of the stuff we can actually look at. Picking on them for not inserting a redundant “known” in every sentence, when it doesn’t add information to their statement and would just be adding filler, isn’t fair.
