We don’t have the water resources to fight fires with controlled flooding, or energy resources - flooding would come with its own ecological problems, affecting everything from plant life to animal to microbial, it would be an environmental disaster even if we had the resources and technology. Controlled burning is also good for nature, nature evolved for occasional burns, it feeds microbial life, it reboots life, it is necessary for many plants to even reseed. It’s cheap. It’s safe when done properly. It’s efficient compared to water. There are many more reasons, but these are the major ones.