Because they aren't the starting QB. It might make a big difference in spring training/preseason or early in the season, but we're in week 13/14 it's really not going to make that much difference. And I'll say again that it's not an exact science. You're trying to find out what gives you the best chance to win. Maybe later in the week they go 100% one way or the other, but you still have to give each enough reps to figure out which way they go.
And for all we know, he may already know and will give one more reps. I don't know how I feel about Rizzi’s long term prospects, but I'm down with whatever. Let him do his thing is how I feel about it.