Don't fire hydrants solve that problem?

Only when present and working, which is not guaranteed.

This is a fundamental problem with emergency services. When a major emergency happens, it happens all at once, and you need a lot of manpower and equipment to handle it. All the rest of the time, when more routine emergencies happen, all that manpower and equipment looks like total overkill, and it is, except for being prepared for something huge.

So, yeah, you don't need a truck that can carry thousands of gallons of water to respond to a house fire when there's a hydrant down the street. But you want that truck to be available in case the shit really hits the fan hard, and since you have it, you might as well use it.

In somewhat rural areas, you see standpipes sticking out of plenty of bodies of water (generally if they are big enough and near the right road).

Not when you live 10 miles from the closest one, no.