An incendiary bullet penetrating a self sealing tank won't necessarily cause a fire. Gasoline needs oxygen to burn. The Allies developed a self sealing bladder that not only plugged holes, but gradually collapsed as the fuel was used up, to suppress the mix of air and gasoline vapor that replaces the liquid fuel as the fuel is consumed and the level drops... in an unsealed tank.
Why is that important? A gasoline vapor/air mix in an enclosed space doesn't burn... it explodes with great force. In the case of a partially empty aircraft fuel tank, enough force to blow the plane apart.
This is why the Japanese aircraft in WW2, that didn't use self sealing tanks, often exploded when hit. A half empty unsealed tank could blow up from a single incendiary hit, if the vapor/oxygen mix was conducive to an explosion (not always, but often). Allied aircraft, that took measures to suppress the creation of gas/air vapor in an enclosed fuel tank, weren't nearly as prone to blowing up with only a few bullet hits.
Even if a vapor explosion didn't happen, and it didn't always happen, the fuel spraying out from an unsealed tank that had been hit could be ignited by anything... the next bullet, electric sparks from damaged wiring, even the flames coming out of the aircraft's engine exhaust... doesn't take a lot to ignite highly volatile gasoline vapor.
Finally, consider what Japan did with its later aircraft designs.
Towards the end of the war, Japan brought out some new fighters. The most capable, and an equal of the better US fighters, was the Kawanishi N1K2 Shiden Kai, which featured 4 20mm cannon, armor plate and bulletproof glass for the pilot, and... self sealing fuel tanks.
Late in the war, it appears that even Japan, the one nation who did not use self sealing fuel tanks on their aircraft, changed their mind.
Additional notes: two somewhat recent air disasters illustrate how the problems of WW2 combat aircraft are still with us.
The hydro shock problem of a bullet hitting a full fuel tank and creating a shock wave was revisited in the Air France Concorde crash. A piece of a tire flew up and hit the Concorde's fuel tank. It didn't penetrate the tank, but the shock wave caused the tank to break open and leak a large amount of fuel, that caught fire from the Concorde's afterburners.
After this crash, the Concordes were given new fuel tanks with a bladder similar to the self sealing bladders of WW2, to keep the tank sealed in the event of hydro shock from an impact, and prevent this from happening again.
TWA flight 800, a 747 that crashed off of Long Island, is believed to have been brought down by an explosion of its center fuel tank. The center tank was nearly empty, and had an air conditioning unit next to it that had been running for some time while the plane was on the ground, heating up the fuel vapor/air mixture. While this was kerosene and not the more volatile gasoline, the nearly empty tank exploded not long after the airplane departed the airport, possibly due to an electrical fault plus the high temperature of the vapor mix.
After this crash, commercial airliners were required to vent inert gas into the tanks rather than atmosphere. Venting with inert gas is also required procedure for oil tankers, after a few of them exploded when a spark ignited the oil vapor in nearly empty oil tanks.