As has been mentioned, "Lookout" means deliberately focus. There are two problems with the human eye. One is that it is drawn to movement. An aircraft on a "constant bearing" (effectively a collision course) does not move, which makes it difficult to spot at an early stage - especially if that "constant bearing" is behind a windscreen pillar.
The second problem is that it is very difficult for the eye to focus on a point in space without a specific target, which is particularly relevant during a lookout scan. This can be overcome (trained to a certain extent) by picking a target (a cloud or a point on the ground) and consciously maintaining your focus at that distance as you move your eyes around. Otherwise your eyes will revert to a focal point outside the cockpit of around 8ft, which is less than useful. Practice is the only way to improve this.
It was also always drummed into me to have a good look up along Finals while I was on Base.