Most refs do "explain" to the goalie before a PK that he needs to stay on the line. Usually, refs pay most of their attention to this aspect of the penalty kick. I never recall a ref warning players about encroaching the penalty area. Probably because they seldom pay any attention to it and rarely call it. The only time encroachment is really an issue is when the shot is blocked or hits the post (particualrly when it hits the post because the PK kicker can't touch it.)
I've warned players to stay outside the 18 until the ball is kicked, quickly as I'm walking by. But look at it this way, if i'm the referee, i've got my far side assistant watching the goal line and my near side assistant back with any players near midfield should a goal not be scored and play continues. I'm standing about fourteen yards out, to the side of the player taking the kick. In my field of view is the penalty spot, the keeper, the kicker (unless he's running up from 20') and my far side AR. Which means that the only way I can see players enchroaching is if by the time the ball is struck they've reached the edges of my peripheral vision. And if that's the case, then they are way, way too close and i'll need to deal with the encroachment.