Most casters would possibly never see much combat because (1) they are extremely valuable and apparently relatively rare and (2) they don't normally have much of a leadership bonus.

It still seems staggering to me that the entire RCC couldn't field a single lookamancer.

I'd also add that we don't know what "leveling" entails. Could a caster choose to spend more "points" on combat ability? There's also no reason to believe that "all casters are popped equal." Some could have inherently greater abilities not only in casting but in other areas. (I just got a lovely idea: Wanda is such a fun character that it makes me want to find out more about her first turns on Erf, her "childhood" as it were... was she more idealistic and less assertive?)