I'm wondering why are most of the aspergers symptoms just symptoms of a brat?

Simplistic. Nobody knows what asperger's is, but the behaviors that you'll see often trouble these kids. They are often jerks but don't realize it, and find the world baffling and often terrifying. Discipline won't help but guidance and explaining kindly may help. By contrast, the real "brat" is likely to be diagnosed with "oppositional-defiant disorder" & understand the impact of his/her behavior well.