Jet fuel can't melt steel beams, but magical jet fuel can.
That was horrible, I'm sorry.
Also, we probably would have gotten our asses kicked to all hell by now if that were the case. Magic and military technology are a very scary combination.
We're pitting Gaps, mountain breakers, magic, sparks, doll armies, Fate manipulation, Time manipulation, and a whole lot of other abilities against conventional military equipment.
We have no methods for suppressing magic, and Gaps themselves are horribly powerful against human equipment. Suika could literally destroy a mountain, in this case an entire city, Marisa's Sparks would obliterate anything in its radius, Doll armies could rampage through cities, Tengu speed is too fast for computers to identify, etc.
So we'd probably be screwed. That's not even counting Flandre and the Lunar Empire, the former of which destroyed a meteor, and the latter of which is a massive nation with technology far beyond our time.
What if Flandre appeared in the room you're in right this instant?