Why is fuel stored in the wings of aircraft?

Why not? You’ve got a lot of space available in there, which otherwise would be doing nothing. The airfoil shape is required to produce lift, but it can’t be solid (too heavy) so it’s a void space, generally.

Provided you drain fuel evenly from the tanks, the aircraft remains in trim and perfectly flyable.

There can also be a tank in the middle of the middle of the aircraft, and even one in the tail.

Julia asks about the proximity to the engines. I’m hoping an airframe engineer can chime in on this. My own view is that you actually couldn’t carry the same quantity of fuel without reducing the capacity of some other revenue-generating zone (cargo, passengers) to a point where it’s not economical to run the aircraft. So you’re pretty much forced to use that space.

So what are the risks of putting fuel near engines? Well there’s the direct risk of an existing engine fire igniting the fuel, and the secondary risk of a mechanical problem (e.g. shedding a compressor/fan blade) puncturing a fuel tank, leading to ignition and fire.

To address the second point first, aero engines are required to contain a shed blade, and must be demonstrated to do so by the manufacturer. So if that happens in flight, it shouldn’t affect any proximate fuel tanks. You might lose the engine, but the problem would be contained. The engine is shut down and the flight continues (the aircraft is perfectly flyable with one engine out) on the remaining engine(s).

The first point, an existing engine fire, is addressed by crew action. If the crew suspects an engine fire (there are detectors in and around the engine), there are immediate checklist items which are performed from memory.

These include shutting down the engine, and turning off fuel pumps to avoid feeding the fire. The crew will use the fire extinguishers and might turn the aircraft to stop wind blowing flames toward the fuselage.

So I think the view is that any problems which could occur can be contained and managed, so that’s not enough of a reason to avoid using the wing area for fuel.