Security guards aren't so much supposed to be the people fighting off a severe threat, but rather just a deterrent. They don't have time to check the bags of every single guest, and they know what to look for to tell if it is just filled with family stuff, or if they are trying to hide something. If something goes down, security guards are supposed to get down and call the police, which is why all theme parks have police officers permanently stationed there. And realistically there ins't much a security guard can do about small bombs or weapons in such a large place without making getting into a theme park be like going through an air port.