I believe that the main reason for that is that people in remission gladly live their life and don't bother to come and tell us about it.

I can promise you that during those 15 years I thought very little about MG (mostly that I didn't think I had it, but regardless I probably wouldn't have thought about it much anyway). Being a physician I saw a few patients with MG during my training, but wasn't more interested in it than in any other relatively rare disease not in the field I was going to specialize in. (you can't know everything about everything).

According to the medical literature spontaneous remissions occur in about 10% of the patients and can last for quite a few years. Another 30-40% (or even more) will have remission with treatment. So, that is quite a lot of people.