Earlier quoted context omitted.
Because, although it could bring down a plane, it's very likely not to. The fire will be detected quickly and then put out. Your question is sort of like asking, if an airliner can fly on one engine, why do they bother putting on two? The answer, quite simply, is robustness.
My point is that it is very unlikely anyone will try. There is exactly nothing stopping anyone from packing an airplane bathroom with alcohol soaked paper towels and lighting it on fire; and yet it has not ever happened. The TSA is a colossal waste of time and money - the more ways that it can be pointed out the better.
It would be fairly easy to put your fire out with a fire extinguisher. But I'd rather not fly with you anyways :)