Good question. There are at least two reasons. First, the idea is to make sure the judicial branch (federal court system) doesn't intrude on the powers of other branches of government - the legislative and executive branches. The courts can only hear certain types of cases and can't issue advisory opinions. Second, the requirement that the court can only hear actual cases and controversies helps prevent the courts from being overburdened. If anyone could bring a case without having suffered actual harm, we would encourage frivolous litigation where no one actually suffered an injury.
