Because he is the best we have (apparently). I have read numerous times that the coaches see these players in practice and knows who can play and who can't.
I do not believe that at all with him clearly being better from the staff. We also saw with Pringle when he come in and took the place of Elam on the line. Pringle is hundred times better than he is but he did not see hardly any clock until Elam got hurt. after he got some clock the whole world saw the truth and the truth was Pringle is a defensive lineman and Elam it's just a body.
so who else is sitting on the sidelines waiting to play that is much better than the starter?