Easily? You ever tried fighting a house fire or a forest fire? Sure, if you succeed in snuffing out a flame while it's still small, that's easy enough, but once it gets going, "easy" goes out the window. And personally I've always felt the show's approach, where Firebenders can manifest fire spontaneously but every other kind of bender needs an external source (at least until Katara figured out sweatbending), seemed contrived and inconsistent. Sure, they managed to justify it somewhat by equating fire with chi energy, but that feels like a mystical handwave to me. The film's approach strikes me as more consistent, even if it does introduce new limitations. There are points for and against both approaches, and I don't feel that either one is intrinsically "wrong."