Basically it is because they are not trained in this aspect of life.
Dr. Weil who graduated from Harvard himself, is a vocal advocate on training doctors in nutrition. Some places are making an effort. Tufts university is one. Another is U.C. San Diego (where Vit D research is being made public) and U.C San Francisco where the Dept of Endocrinology is researching obesity and nutrition.
This video comes from U.C. SF and is a lecture being given to medical people there:
http://www.youtube.com/watch?v=dBnniua6-oM

I myself discovered many nutritional things myself, when we started with our first computer over a decade ago. I search and learn every day. Most doctors will not take that time, and also have internal prejudice about the internet. Many specialists do not like having their patients search the net for answers. Some very special ones like you see on Mystery Diagnosis on Discovery Health TV, are the exceptions. That program illustrates very painfully how long some patients go before being diagnosed correctly. There is one segment of a woman who went 10 yrs and saw many doctors and not one diagnosed her low B12. That is a sorry example of the quality of medical care in this country.

You have every right to be grumpy and angry! I get that way too!