Because to indefinitely maintain perfect water parameters yourself, without the benefit of water changes or Nature, would require the capability to test for every element and chemical compound, a full understanding of them all, and the ability to add or remove them selectively at will.
Nature will gladly help, but she doesn't have the same definition of "perfect" as you do. To her, perfect means only full of the greatest possible amount of life, and she doesn't much care if that life is plant, algae, or bacteria.
"Look at that front glass," she says, shaking her head disapprovingly. "How utterly barren! It would look so much nicer with a verdant coat of green dust algae. And this excess iron is just wasted, when it would be perfect
for growing a bacterial colony of Thiobacillus ferrooxidans. I can hardly wait to see it swaying gently in the current!"
Yup. She'll do that kind of interior decorating, or something else you wouldn't find acceptable.
So she needs an occasional push in what we
consider the right direction.
And that's where, amongst other things, water changes come in. But if you wait until something starts going wrong to perform the change, it causes a larger disruption than if you'd been doing regular preventative changes all along.
Depending on the circumstances, water changes don't necessary have to be frequent, large, or burdensome. But they're still a good practice.