Because players like Dukes, Rasmus, and Upton have already dominated every level of the minor leagues and have nothing left to prove. I imagine that Dukes and Rasmus will probably be starting everyday fairly soon. Austin Kearns is a holdover from the Jim Bowden era and is washed up. I would guess that Rasmus will eventually replace one of Ankiel, Duncan, or Ludwick. The Cards could probably trade one of their outfielders to get more starting pitching or bullpen help.
Plus for whatever reason, I think managers have a strong tendency to favor veterans over young players.