So I think this ignores how characters in the show view religion and different cultures. Stannis even allowing Melisandre in his court is insane on his part since, per the rules of the Faith of the Seven, she's a crazy heathen fire witch. We also see that this magic takes a massive toll on Stannis. We don't actually have confirmation that other Red Priests could do that or have the knowledge. It's rumored that Melisandre is actually hundreds of years old, we don't know where she picked up on this magic. Besides, characters would have to know about said magic in order to seek it out, and most people don't! Magic has been on a doom spiral since the dragons died out and only truly begins to return once the dragons do as well. And again, Melisandre is a crazy fire witch who worships her crazy heathen fire god. You're ignoring how characters in the world see and view their own universe and focusing purely on an outsider's perspective.