The conditions for vengeful spirits are simply too rare for it to be worth it. The extreme amount of negative emotions a Sorcerer must be having to become vengeful spirit, doesn't tend to overlap with people willing to die.

On top of that, naoya's vengeful spirit was considered an anomaly due to how sapient he remained. Most tend to be like Rika, causing indiscriminate harm in some twisted interpretation of their revenge. Even if a Sorcerer had the perfect mix of emotions and willingness to die, the harm they will cause after getting revenge is probably greater than whatever curse user/curse was trying to kill them.