Steve could be remaining silent for a variety of reasons. One thing that readers are always left to wrestle with is exactly how much blame Steve Harmon deserves for his actions or inactions. Myers rarely shows readers Steve being clear with us about his own emotions and reasons. Steve himself even questions whether or not he is a monster. Part of his questioning stems from the fact that he does know and hang out with people like Bobo and King. Steve somewhat reveres these characters. On the other hand, Steve also fears them, because he has a working moral compass. This is why he chooses to be a superhero, rather than a villain, when his brother asks him about it. Steve might stay silent because he does not want to contribute something dumb to the conversation and have these guys laugh at him and think less of him. Steve doesn't have experience with robbery and making these kinds of plans, so remaining silent is the safer option. He gets to be "cool" and tough by hanging out with these people, but he does not risk showing his actual naivety by saying something.

Another possibility is related to his film-directing passion. Steve sees himself as a filmmaker. In order for him to show audiences the reality of things, he himself has to be an observer. Staying silent means Steve doesn't taint the scene in any way. A third possibility is that Steve hopes to be left out of the plans. By remaining silent, Steve won't get pulled into the plan.