Most of the top QBs every year don't work out at the Combine. Some don't even work out at their pro day. All it can do is show things to get nitpicked and hurt your draft position.
Lower rated guys do it in hopes of impressing teams and moving up.
I'm not a huge fan of Sanders game since I think he holds onto the ball too long and doesn't have a great arm or pocket awareness but he's either the #1 or #2 QB in this class so not much point for him to work out at the Combine.
Plus, much like the Mannings, the Sanders family is likely to refuse to let him play for some teams so he's essentially going to choose where he plays anyway.