Try this experiment. Focus your attention on the cross for a while. As you concentrate hard to focus on the cross in the centre, the background slowly begins to disappear.

This could be a possible explanation why crew of AC759 flew over 4 aircrafts at San Fransisco with noticing them.

The Troxler Effect is named after Swiss physician and philosopher Ignaz Paul Vital Troxler (1780-1866). In 1804, Troxler made the discovery that rigidly fixating one’s gaze on some element in the visual field can cause surrounding stationary images to seem to slowly disappear or fade. They are replaced with an experience, the nature of which is determined by the background that the object is on. This is known as filling-in.

Read about the troxler effect from the website.