Comment by WORLD_ENDS_SOON

I think it is because many labs in CS departments do very little research involving human subjects (e.g. a machine learning lab or a theory lab), so within those labs there isn't really an expectation that everything goes through IRB. Many CS graduate students likely never have to interact with IRB at all, so they probably don't even know when it is necessary to involve IRB. The rules for what requires IRB involvement are also somewhat open to interpretation. For example, surveys are often exempt depending on what the survey is asking about.