A) he's looking to improve his draft stock. Another year at a higher profile University, one that runs a more pro style offense, should hopefully help that.
B) He's got a number of NIL deals. It's conceivable that he'll make more in CFB next year than he would as a 6th round NFL pick.
It's a bad question anyway because who cares if Hartman is an NFL QB or not. The right question is, does he make the ND QB room better. The answer is, unequivocally yes.