Why do Phobia exist if they aren't advantageous?
I think the fault in your reasoning is the assumption that phobias are not advantageous to us. As an example of a potentially useful phobia, consider the fear of heights. Now consider two different ancestors: one who was afraid of heights and one who was not. Generally speaking, the ancestor who was afraid of heights would likely avoid such environments and thereby reduce the likelihood of, say, falling off a cliff or high precipice. In evolutionary terms, we would say that natural selection confers an advantage to the phobia-possessing individual(s). This website in particular has some interesting reading:


  Others believe that phobias exist because of evolutionary development to avoid danger in order to improve survival. There are several stimuli that are shared by the most phobic people (heights, insects, crowds, etc.) These phobias, with a couple of exceptions, relate more closely to pre-technological societies. Thus, some scientists, such as Martin Seligman, believe that people are inherently "prepared" to fear certain objects. ((5))) In other words, our phobias relate closely to the perils of our ancestors, as through natural selection, those who feared the dangerous stimulus survived while those who didn't, died off. Prepared fears appear to be very easily conditioned. Some scientists, such as LeDoux, believe that preparedness and the ease of conditioning are the result of certain preexisting neurological connections that exist evolutionarily. These connections are turned on with relative ease.