Inherent to the D&D system, all the way back to the beginning. In the original version, 1st level mages could only cast ONE spell per day, and had to chose it and "memorize it" in the morning, so could not change their minds later.
As for backup Archers, that's a bonus too, as weapon restrictions in the early versions meant they could only wield dagger, sling or staff.
I think the idea was that "magic" was so inherently (potentially) super powerful it had to be limited a LOT.
