Some never do focus on it. There are some indeed who never look back and just keep going straight. But that does not mean that there are not those who do. Or, that it cannot happen. Those are the people Paul is addressing, the people who were struggling with the two natures.
I in no way mean to say that the born again believer is not able to sin, nor do I pretend that they are free from the temptation to sin. However, I will say to my dying day that the born again believer has been set free from the servitude of sin. Before a person is born again they have no choice but to sin, they are enslaved by it.
I know of quite a few people who focus on sin. But I think that we may be thinking about different definitions of this.
The analogy is like this;
Archery is a wonderful exercise of self-control, focus, and skillful training of ones body. When an archer draws and releases an arrow, there is much more to it than pulling back a string and then letting it go.
The master of archery is one who practices it every day. And not just every day, but throughout the day. It is something that you make apart of yourself. When an archer gets ready to shoot, the bow and arrow become an extension of who he is.
However, just like with anything else, you will never find an archer who even after spending day after day - year after year - will hit the mark exactly in the same spot he was aiming each time he shoots. In fact, archery is always based on the moment by moment time that you are shooting. And because no two moments in time and space are exactly alike, there is no way to ever prepare for anything that might happen.
So, the archer draws, anchors, picks a spot(mark), and releases. What happens? More often than not he misses the mark. But what comes next is what sets the amateurs apart from the true archers. The amateur looks at the spot he did hit and judges from it where he should aim next. But in doing so, he sets himself up for a unending cycle of never hitting the mark. Because he is not expecting to hit the mark he sets for the next shot, he is expecting to hit the mark he was looking at the first time. However, because no two moments in time are the same, he has ruled out any wind variation, humidity changes, changes in arrow weight, changes in draw length, changes in the anchor point, etc. The list goes on and on.
Contrast that with the master archer. He misses the mark on his first shot. What does he do? He forgets about that shot. He concentrates harder on the mark. He may very well miss again, but each time he shoots he concentrates harder and harder. He is able to do that because he never looses sight of the original mark.
The amateur cannot concentrate hard on the mark because the mark he looks too is always changing based upon where he hit last. Oh sure, he can see the original mark, but it does not mean as much to him as it does the master archer.
This is the Christian walk illustrated. There are many 'immature' Christians around because they do not keep Christ as their one and only mark in focus. They walk along, they sin, they look at the sin and say "I don't want to do that". So they look at that sin, examine it, see what they did 'wrong', and then try to avoid it in the future.
The fallacy in this is they use their judgement in trying to figure out what they did wrong. There are so many things that it could be. And even worse, a lot of the time they will look to others to see what direction they should go in to keep it from happening again.
But the mature Christian looks at the sin he commits and says, Jesus. Then turns from it(repentance) and never looks back. That is what repentance is, it is a turning away from. And if you turn away from something, tell me, how can you still see it?
Keep one focus, and one focus only. Jesus the Christ...His righteousness imputed onto you. The more this is your focus, the more times you will hit closer to the mark. But when one associates themselves with being a sinner, then they are associating themselves with the natural man and not the spiritual. These two are contrary to one another and cannot be focused on together. Its one or the other. And like it or not, you have to choose between the two.