These things don't necessarily follow one from another. Just because many fans are casual players, it doesn't mean they don't have a liking for the genre. D&D is not inherently repealing to the same group either. As much as Baldur's Gate targets an adult audience, Pokémon as a series is over 25 years old, there is a sizable number of adult players. Not only there is an overlap, the differences may make it appealing to players with differing interests too.

Mind you, I am an adult Pokémon fan who plays D&D. As much as I understand that not everyone is like me, it's not like Pokémon fans all evaporate when they hit 18, or they never again care for turn-based games.

Ultimately, the success of the game vindicates that there are people who want that.