I suspect mainly because you don't and can't understand the world; you understand whatever simplified model of the world is inside your head and by default it's real unobvious just how simplified exactly.
Pretty much. We just make the assumption that we can hold a model of the world in out head.
But like that old axiom about models goes 'All models are wrong, but some are useful.'
The taoists would argue that you shouldn't try to define everything because it will not help out after a certain point.
At least recognizing that we hold simplified model of the world, and knowing that it is limited, is a reasonable place to be in and that helps to explore more.