Ok, I am going to bring in Paley's watch here for a moment. Paley's watch argument comes up in various forms (ID is based around it), because people have witnessed personally that complex systems that work together too well are engineered. People see what men do over and over, and then when they see something else that no one who made it is a sign of, the initial belief is that some intelligence engineered what they saw. The default position is that the item in question is the byproduct of intelligence, and that is the acceptable default position.
Ok, now bring on people who are evolutionists, and particularly non-theists. Such individuals end up trying to argue with people the default normal view one should take when one runs into complex systems that look like they were engineered, is that they weren't designed, but came about by a mix of unguided deterministic and random changes. And the changes that remained were the ones that survived a hostile environment and ability to change.
If you want to know why, among normal people, evolution as THE all encompassing way of life, that doesn't require a intelligent creator isn't accepted as the norm, it is because of this. The norm for people is that complex systems are engineered, and not the byproduct of unguided forces that are a mix of random and determinism. We actually have almost no actual lab experiments that fully functioning complex systems that work together aren't engineered. Even in experiments where there is chaos involved, and computer models, we tend to see that people code the program to run the environment and set parameters.
