Firstly, the keeper missed it by a rather large fraction. Also, Rooney decided to avoid contact - the keeper made no attempt whatsoever to withdraw his tackle when it became clear he couldn't reach the ball first. At the very least it was 'careless' which is a foul. In my opinion it was 'reckless', which is a foul or yellow card.
If you make a sliding tackle and miss the ball and man, the winger probably has more incentive to stay on his feet and attack the space which has a greater chance of resulting in a chance than from a wide free kick with all men back. If your action impedes his run/balance then it would be a free kick/advantage regardless of whether you touched him or note. When in the box, the advantage to the attacker is to go down and take the penalty.
as a keeper who has played to a decent standard it's pretty hard to pull out of a sliding stop once you are committed - its part of our game and one thing we have that can help us out when an attacker is advancing into the box! Rooney had the opportunity to jump the slide and remain on his feet - he chose not too!