This world must not have much of a banking system. She could be a top class hunter for a year and then just retire.
While she could become the top class adventurer but she don't want to. All she want is just a regular life, stable job working 9am - 5pm, relaxing at home after work, eat, sleep and then repeat.
If she become adventurer, sure she can be the top very fast but that mean she has to fight with monster regularly, which she doesn't want to live like that.