Why? Because few doctors have much knowledge in nutrition and also in the use of vitamins/supplements in healing... You probably get more education on this board listening to some of the people here who are extremely knowledgeable in these areas...
I may get blasted for this, however, prescribing vitamins and such do not make drug companies any money... Doctors only prescribe what they learn (from the cute little drug reps in the mini skirts)

There are no cute little reps going around from the vitamin and supplement companies, because most of the results in the use of vitamins/supplements are not supported by studies done under strict scientific regimens...
I do think that some of the younger doctors are beginning to pick up on some of this though. I had to go to Urgent Care a couple of months or so ago. When the doc went over my list of meds (croak), he said, "You are taking this B12 for pain, aren't you?" Faint!!! There have been a couple of others who have recognized a connection between the use of B12 and pain.
I have questioned why the use of oral B12 doesn't seem to help my pain as much as it used to. The one thing I always forget to include in my health history is the presence of parietal cell antibodies in my bloodwork. These are cells in the stomach which provide "intrinsic factor" which promotes the absorption of B12. Maybe I just am not getting enough of the intrinsic factor to get oral B12 absorbed, I don't know.
One research doc thought perhaps, with the autoimmune issues, it might not be processed properly. I take injectible once a week and it helps a lot. I have read too much of Rose's B12 work though and STILL take oral regardless. As someone posted, too much is not going to hurt you. The excess is eliminated in urine...
Cathie