Because on some people, they just do. There's nothing weird about this, no special explanation.
Actually, the "philtrum" is the indentation in the area between the bottom of the nose and the top of the upper lip. This area is itself often referred to as the upper lip. An indentation by definition can't "stick out," so I think you're referring to that upper lip area.
In some people, it's just very prominent, sometimes because the upper teeth are prominent and large.