Because of Gojo Saturo, an incredibly strong and impressive sorcerer of the Jujutsu Kaisen universe. His interest sparks the moment Gojo realizes that there's an individual who can keep Sukuna contained.   Therefore, he wants Yuji to keep eating the fingers and keep him trapped in his body. If he can do that, there's no punishment for him. But if not, then Yuji will be executed, killing Sukuna in the process.