Only a few are. An example of a complex system would be a stream flowing into the ocean eroding the sand as it flows. Many factors combine to determine the stream's path. By placing rocks in certain spots in the stream the actual course of the stream can be changed. A small thing like a rock can create a big change in the final result.
Small changes producing big results is one of the characteristics of a complex system.
The difficulty is finding out which of these small changes will produce big or breakthrough results. Another characteristic of complex systems is that these breakthrough or critical change points are not easily identified. Fortunately the concept of continuous improvement helps out. For example, if we assume a business analysis shows us 50 things can be done to make business improvements we know that some of the 50 things are more important than others but we really don't know much about the actual priorities. We could do more study and see if we could understand the priorities better.