I am relatively certain Sark is as baffled by Quinn Ewers as the rest of us.
I tend to think he really believes in his players and what they can do and when they don't live up to it, rather than bench them, he gives them more that they should succeed with, hoping they snap out of it and execute at the level he believes they are capable of doing.
I imagine Ewers is quite a practice warrior, because serious contact is off the table and I am pretty sure now that he is officially contact-shy.
The plan all along was to slowly develop Arch until he is fully functional with the playbook and not to rush him into game situations until that time, unless absolutely necessary. So that means he's not quite considered ready, despite the advantages over Ewers, real or perceived.
So, it's not bloody-minded stubbornness, per se, although it appears that way. There's a method to the madness, for better or worse.