There isn't two better options on the bench though.
Milner is there because we play 2 up top. Walcott is a very good player, but one who is best for counter attacking football and doesn't offer much defensively. Milner tracks back very well, and positionally he's top notch. He may not be the best footballer in the world, but he's important to England's style.
If you wanted to play Walcott instead of him, you'd have Walcott, Young, Welbeck and Rooney and two 31 year old midfielders (one who is the worst player in an England shirt currently, and one who is known for being positionally suspect) Milner offers a lot of reliability down the right side, and to be honest if the keeper hadn't have made a great save from Cole's volley, would have had an assist this tournament.