Quick question on Mason Foster

I understand the concept of Lovie's scheme and how he wants a Mike that can cover the deep middle, which is why he doesn't believe that Mason is a great fit.  What I do not understand is why they don't just move him to SAM instead.I rather Keep David and Foster while trying to find a suitable Mike.  He's still young, has been productive, and won't require the range/speed needed to play the SAM.  I understand how Lansannah had a pretty good season at SAM, but I still would much rather have Foster due to his age and physical presence.  Anyone have any insight on this as to why there hasn't been much talk of them keeping him to play SAM?  I could have swore that back in college he played some SAM.