To answer this question, take a look at how Macbeth reacts to each of the prophecies in act IV, scene I.

When Macbeth is told to beware of Macduff, for example, he thanks for the apparition for its "good caution." In other words, the prophecy confirms his paranoia about Macduff, thereby creating a sense of security in Macbeth.

With the second prophecy, Macbeth is told that "none of woman born" can hurt him. This creates a false sense of security in Macbeth because he believes that he cannot be hurt by Macduff. However, he decides to kill Macduff anyway because it will make him sleep easier at night. What Macbeth does not realize, however, is that Macduff is not "of woman born" because he was delivered by cesarean section. This leads Macbeth to dramatically underestimate Macduff, as we see later in the play.

Finally, with the third prophecy, Macbeth is told that he cannot be destroyed until Birnam Wood comes together with Dunsinane Hill. Because Macbeth believes that the movement of these two places is physically impossible, he becomes confident in his ability to remain king.

What we find, then, is that each prophecy encourages Macbeth to believe that he is invincible; that nobody can remove him from the throne of Scotland. As we will see in act V, however, this sense of security is false and will lead to Macbeth's demise.

As each prophecy passes and ultimately comes true, Macbeth is bolstered by his sense of immortality. No man born of woman can harm Macbeth and no harm will come until Birnam Woods marches to Dunsinane (seemingly impossible things) all give Macbeth the feeling that reality will never come, that he is indestructable and omnipotent. He chooses to ignore the prophecy of "Beware Macduff."