What I'd like to know is why can't they make an "anti-fire bomb" that could be dropped from a conventional warjet?
Modern warplanes can certainly carry far greater loads than WW2-era warplanes, but the big problem with water is VOLUME. You need to be able to put the stuff somewhere. Modern warplanes are incredibly-densely packed with engines, fuel, and electronics. There's nowhere to put the stuff, internally. And hanging it under the wings, you'd reach the maximum size of a container that would fit long before you get anywhere near maximum load-carrying capacity.
So you're stuck either using a bomber, and putting the water in the bomb bay (but here too, capacity is not as great as you might think: modern bombers rely on high explosives and precision for their punch, not on sheer volume of explosives) or on transport planes, which by definition have high internal volume available to haul things around.
So while it's fun to think of an F-18 divebombing a fire, it's simply not a practical solution to the problem.