That is because your claim is just not true. It can be demonstrated in computer models that you can get complexity via random variation, simple rules, and a filter of selection.
YOU seem to are unable to get beyond the concept that most things that are designed by people do not have a very specific attribute to them. That is, they are not self reproducing , self catalysing chemical reactions. When you have that, plus the filter of natural selection, you have complexity being formed.
You don't need a designer for that.