Omar isn't known for being an idiot. He wasn't always able to rob people on his name alone, in the beginning of his career (and until the end of it on difficult jobs) he would undertake in-depth surveillance of the place he was going to hit so as to formulate a strategy to come out on top. And guess what? He's pretty damn good at it. If you can rob the most powerful people in the hood and walk away, who knows what you're capable of. Also, Omar works with other people quite a bit, so if you can only see him who's to say there aren't more people who are prepared to kill you and take your drugs even if you take out Omar? Is your own life worth more than drugs that don't even belong to you?

In war, information is much more powerful than bullets. Omar has used it in his past to rip off big names, and he uses the corner boys lack of information to play mind games with them. You have to realize that the corner boys aren't watching the show with you, they don't have any insight into Omar beyond what they've heard on the street. And on the street, his reputation makes him one of the most dangerous people to mess with.