Maybe it's because of how long ago it has been since the last Sage was named that the standards of the title that came with it don't come to par with the current trends in this series' setting?
To go into it further, yes, they may be able to cast all kinds of magic, that's an achievement of itself, but what about how they can be applied practically in real-world settings? What about further continuing the advancement of magical research? What about making a great contribution to society as a whole with their magic? We don't know if the Sages of the past were capable of that much in feats.
To me, getting the title of 'Sage' here seems like it is almost no different to getting a Bachelor's from a community college. You got the education and practical training in the field of your interest, certainly, but now you must step into the real world, get a career, and make something of the education you painstakingly gathered, something that more than 60% of the American population has failed to accomplish within the last decade or so (myself included).
It's really no different with these girls who could have learned most of what they need of magic from the books they've read and just become adventurers the long way by getting old enough to register, much like how a truck driver only needs a high-school diploma and a certain driver's license to work as one.