While there certainly is widespread misunderstanding and ignorance about social anxiety, the same is true of most psychological disorders, and plenty of other things like race, nationality, and religion.
I've grappled with a lot of such ignorance in my own life, but have found that it isn't very productive to feel resentment towards people for not "getting it".
For one thing, there the possibility that you're equally as ignorant about some aspect of their life. Few people have an encyclopedic knowledge of mental illness, and even some psychiatrists, whose job involves having that kind of familiarity may not understand a particular disorder if they've rarely encountered it in their work. Put simply: most people have no reason to know any better.
There's also the problem that we're usually limited to very subjective descriptions of what it's like. Sufferers of the disorder tend to have a second-hand understanding of what "normal" feels like, so it can be very difficult to relate. (Many of us have had this familiar conversation: Phobic:"I feel uncomfortable in situation X." Non-Phobic:"Oh, me too. Everybody does." Phobic:"No, I mean I feel really uncomfortable.", etc.)
Objectively, there are studies and statistics, which don't really convey anything about the experience of the disorder, and there are diagnostic criteria, which involve such vague wording that people can find confusing. I've found that sharing these with people is best done in combination with describing your subjective experience.
Social Phobia can be particularly confusing to some if they see you as being fine in some situations but not in others. For example: Where once I found it impossible to order food in a restaurant or approach a counter in a store, now I do it readily. Yet I still consider myself phobic because there are still huge areas that I find extremely difficult or impossible. To an outside observer it may seem very selective or inconsistent. It would be wrong to think that they should understand that easily.
Finally, I think it's important to remember that it really isn't that important for people to understand everything about your disorder. It's your problem to solve, and helpful as others can be, the most important part is yourself. I know when I was younger and just starting to understand my issues I desperately wanted someone to understand exactly what I was going through. After years of feeling like people just didn't get it, I realized that I was getting a lot of value out of trying to explain what I was going through, regardless of whether I felt like they understood. Everything I put into to words was something that helped me better understand myself, and that's the real trick to helping myself.
