I agree with post 2. Shakespeare is tying up loose ends, announcing Malcolm as king. Restoration to the throne is one reason for continuing the play. Also, it is a moment of celebration. Macduff has his vengeance. He is carrying the head of Macbeth. He is triumphant and he wants everyone to know. Although Macbeth's death does not bring back Macduff's wife and children, he does at least have revenge. Macduff cruelly separated Macbeth's head from his body. That had to be a victorious moment for Macduff. He wanted to celebrate.
Well, concerning why Shakespeare continues Macbeth after Macbeth's death, he doesn't continue it for long.
The short answer is that Macbeth's death has to be announced. The violence occurs off stage and the audience must be informed as to the winner of the fight between Macduff and Macbeth. The two exit the stage fighting, and who wins has to be revealed. That's why there has to be a conclusion.
Also, though, order must be formally restored in Scotland. The play is a tragedy, and things must be set right, the evil must be officially destroyed. Malcolm has to be announced as king and loyalty declared to him by the thanes. The evil is destroyed and order restored in the conclusion.
Shakespeare also uses the conclusion to tie up a loose end: to reveal Lady Macbeth's death as a suicide.
All that said, however, you should be careful of attempting to speak for an author. My answer above gives you the function and purpose of the conclusion. I can't tell you why Shakespeare did anything. I can only tell you the way the literature works.