What's wrong with I don't know? We don't like the vacuum it creates. The uncertainty it leaves us with. It makes us nervous. So we make stuff up. And then we make all sorts of assumptions and plans around the made up stuff.
What a waste of time and what a waste of energy to keep convincing yourself and others to believe in something you made up. "I don't know" may be uncomfortable but it sure is easier and allot more reliable than pretending.