Because all a mage needs to be powerful is to be born with it, maybe read a book and occasionally drink a potion.
It's the only power fantasy they can see themselves in because it seemingly takes the least actual effort to achieve 'power', no need to pump iron, no need to do push ups or train martial arts for years, nah, mages are "born with it all" geniuses who just talk words and swing sticks.
Also mages are nerds of the nerd world, loners locked away in their room studying ancient texts and conduct experiments and they're usually utter egomaniacs and narcissists.
It just translates easier for people like Narcissa, he's a total nerd with a singular obsession, locked eternally in his room reading some dumb crap and doing 'arcane experiments' like his stupid video game shit. Totally a mage.
I know some people who always play as a "mage" in video games and they all fit my logic behind it.