Just wondering why don't they believe it ?
This is probably because in some places, like in the U.S., doctors are treated as if they are something special. This is probably because so many of them make huge sums of money. We all know how many folks in the U.S. think money rules. It takes a lot of education to become a doctor. We also tend to admire the highly educated. However doctors are still just regular people, like you and me.
I suspect some folks don't believe you are a doctor because they cannot imagine someone of that profession spending time shooting the shit with a bunch of morons in an Internet forum. Incidentally, what kind of practice do you have?
The arborist I hire to prune my trees each year has a Ph.D. There are many types of doctors in the world.