One possible reason is learning a spell takes time. Like The Tourist After a decade or 2 of study he's still pretty feeble at dragon magic, not even up to apprentice level yet. Though he is not going to one of the academies on Korodos to study magic and his part time tutor is Sara who may not be really qualified from some folks viewpoints who are ancient.
Logically the spells the others can cast should be limited to only certain spells and also logically they should be inflexable. If they can do a fireball they are always the same. A full mage can do a custom one for the situation. Or a light spell. They can't custom it, it's either on or off. They should't be able to reduce it like Sara can a light spell and reduce the area that is lit and switch the ongoing spell so it now focus forward like a flaashlight, change it's color, switch it to only infrared light if most have infrared vision or goggles, have it flash in certain colors as a signal, ect. Like one can't do custom bullets to say delay to explode. One that can move to the left as a foe dodges to the left, switch target if you decide to hit another target, ect. Though the plus is a spell from an item does not tire the mage any, if spells do tire mages, and lets them focus on a spell needed for the moment.
Though that is up to the gm and how magic works, just my insight on how things work when I use to write fantasy.