Maybe he just isn't a finisher. Giggs only scored like 1 in 9, Di Maria and Ribery have like 1 in 6. Saka isn't Mbappe, leave him be and let him do what he's good at without expecting him to be able to do everything really well.
He is still too young to make that sort of assertion. A guy like Rashford hit the ground running and was scoring consistently when he broke through at 18. He went through a dry spell afterwards and a couple of seasons of inconsistency, poor decision-making and generally selfish play. Saka may end up like Di Maria, but he can also end up like Sadio Mane. I believe he has the talent to become a capable finisher, he just needs more reps and composure. All those things come with experience.