A fireball would certainly be problematic, and fires are very bad for planes, but only when they are less controlled than in the passenger cabin.
You would be very unlikely to be able to create an explosive, since there's not a lot of chemistry supplies for sale, but it's trivial to build/buy a weapon to attempt to take over the plane, and the threat has been demonstrated at least once that I know of.
Like almost all airport security since 9/11, preventing people from bringing weapons onto a plane is almost meaningless. Sure, you could hurt and kill passengers and flight attendants, but you could do that kind of terrorism anywhere. It won't bring down the plane, and as long as the cockpit door stays closed you will never hijack the plane.