I get what you're saying Kurai, and since the show doesn't explain anything, there is little I see coming from an argument. I find it unlikely that a world with gelling agents [fairly certain I saw some candy that would necessitate this, maybe not] and petroleum with modern sensibilities would not produce a burning agent to kill forests, but we can assume they do not have one. We are told that grimm get more powerful the further they are from civilization, and the only one's we've seen have been mediocre at best. I find it hard to believe they can be a big enough threat to require something besides man-portable crewed weapons, but it can be assumed they don't have them.
I'm coming to this thinking about works of fiction like World War Z, or the very real Second Boer War, or the removal of the Mongols from Russia, in which modern technology and competent organization overcomes massive numbers and more deadly 'gimmick' capabilities, but we may very well just assume that people in RWBY are fools who refuse to pursue useful technologies and Grimm are secretly a credible threat to an airship fleet numbering hundreds of warships.
And I do, for this RP. I just think it silly. Things can be both dumb and good at the same time.
TL;DR: Explanations are contrived, but I don't care, argumentation at this point is pointless.
EDIT: Fuck, I mentioned UBER. Time to re-read the whole thing. 10/10 best comic books in a long time.