This is one of those questions that make you go Hmmmmm. Read this adaptation from Matthew Henry's Commentary on the Whole Bible. I read it today and it makes perfect sense to me, and gives me a very logical response for the skeptic.
Why Did an Angel Roll Away the Stone?
The angels frequently attended our Lord Jesus: at His birth, in His temptation, in His agony. But upon the cross we find no angel attending him. When His Father forsook Him, the angels withdrew from Him, but now that He is resuming the glory he had before the foundation of the world, the angels of God worship him.
The angel came, rolled back the stone from the door, and sat upon it. Our Lord Jesus could have rolled back the stone Himself by His own power, but He chose to have it done by an angel to signify that having undertaken to make satisfaction for our sin, He did not break prison, but had a fair and legal discharge, obtained from heaven. He did not break prison, but an officer was sent on purpose to roll away the stone and open the prison door, which would never have been done if He had not made a full satisfaction.
But being delivered for our offences, He was raised again for our justification. He died to pay our debt, and rose again to gain our acquittal. The stone of our sins was rolled to the door of the grave of our Lord Jesus (and we find the rolling of a great stone to signify the contracting of guilt - 1 Samuel 14:33), but to demonstrate that divine justice was satisfied, an angel was commissioned to roll back the stone. The angel did not raise Him from the dead, any more than those that took away the stone from Lazarus's grave raised him, but by this he intimated the consent of Heaven to Christ's release, and the joy of Heaven in it.
The enemies of Christ had sealed the stone, since this was their hour, but all the powers of death and darkness are under the control of the God of light and life. An angel from heaven has power to break the seal and roll away the stone, though ever so great. Thus, the captives of the mighty are taken away.
The angel's sitting upon the stone, after he had rolled it away, is very observable and shows a secure triumph over all the obstructions of Christ's resurrection. There he sat, defying all the powers of hell to roll the stone to the grave again. The angel sat as a guard to the grave, having frightened away the enemies' guard; he sat, expecting the women, and ready to give them an account of Jesus's resurrection.
Adapted from Matthew 1:1 Henry's commentary on the Whole Bible.