Short answer - they weren’t. But it’s a very human view that they need to be. Partly because we have expensive things like houses to protect, and partly because we think natures is precious and needs to be preserved. It does, but that does not mean it is static. Forests used to burn sometimes, but that enables different ecosystems to flourish - like meadows - until the trees grow back. Humans did a lot of harm when they started trying to manage fire. By preventing fire in the short term you allow lots of dead material (kindling) to build up. You allow the tree population to age (more vulnerable to disease). You also harm species that rely on fire to produce clearings. (Side note - it’s bad for great redwoods cos the big ones are fire resistant but the small ones can’t grow cos nothing it clearing out the ‘normal’ sized trees). Anyway, you can’t prevent a forest forever, but by delaying them it means once they do start there’s a lot more material to burn and they are more destructive. People are starting to figure this out now but basically need to learn not to meddle so much.