2. Racial segregation has made the divided kingdom weak
There are a handful of core races in Hyrule. Hylians, Kokiri, Gorons, Gerudo, and Zora would be the main ones. But they rarely ever mingle. Despite “unification,” each people has its own distinct area of the kingdom (central towns, forest, mountains, desert, and rivers, respectively) and they by and large stay there. The almost entirely female Gerudo have a distaste for Hylians and only mingle for breeding purposes. The Kokiri enforce their isolation via a (false) collective cultural belief that they’ll die if they leave the forest. The Zora have the excuse that they get ill if they leave the water for too long, but it’s notable that they generally remain at the head of the river, far from central Hyrule.
And thus, every single time Ganondorf appears, the various factions’ lack of cooperation means that a small boy dressed in green has to save the world. He does so by visiting each region in turn and helping out each civilisation with its various problems before using the rewards to do similar elsewhere. If Hyrule truly worked together as one nation, combining their various gadgets, tools, magic, and knowledge instead of waiting for Link to collect it all, Ganondorf would be a mere irritation. But they don’t. Segregation is bad, kids.