Why? Because the narratives and the letters of Paul suggest that the visions of Jesus were not originally connected to the empty tomb stories. The claim to visions of Jesus were not the same as the claim to the empty tomb. The two are merged in the gospel narratives. You can see how the two claims are woven together nicely in the Lukan narrative where you have a Petrine vision of Jesus which is separate from the empty tomb narrative but edited onto the story about the empty tomb. You have the empty tomb story that has been further embellished with the vision-eucharist story of the two on the road to Emmaus; and you have the confession of the eleven in Jerusalem that Jesus had appeared to Simon, a vision that has nothing to do with the empty tomb at all. We also have Paul's report that Jesus first appeared to Peter (nicknamed "Rocky"), an appearance that has nothing to do with the empty tomb narrative.
So the empty tomb is a later story that developed in order to offer an explanation for a christology that was beginning to ferment in the earliest community after Jesus' death. When we examine how the tradition came into being, it looks to be that the original claim to a vision was Peter's. It may be that we also have an original claim to vision by Mary Magdalene as well, since John preserves an interesting line: "Mary Magdalene went and said to the disciples, 'I have seen the Lord.'" This claim to vision has been attached to two empty tomb stories, one in which Mary finds the tomb empty, and the other in which Peter and the beloved disciple (=Lazarus), find the tomb empty. Paul doesn't appear to know the Magdalene claim, just as he does not know the empty tomb story (which isn't the same thing as resurrection from the dead, which I will address shortly).
What was Peter's vision, and Mary's vision? Peter's vision is never related to us in the narratives or Paul's letters. All we know is that the Lord "appeared" to Simon. The Johannine author transmits an elaborate story of Mary going to the tomb and seeing Jesus there in an unrecognizable form. She mistakes him for a gardener. Since the later empty tomb stories all have Mary at the tomb, and John ties her vision to this visit, I think that the Magdalene claim to vision (whatever the vision actually was) may have been a claim to have seen him when she visited his tomb. Like Peter, the claim itself was that Jesus "appeared" to her.
It is not easy to piece together what might have happened. All we have recorded is what they thought happened, or better, how they interpreted what was happening to them. I have blogged on the resurrection of Jesus before, so this is not new news to most of my readers. I maintain that Jesus' physical dead body was not raised. This is not what happened, although this is one of the interpretations of what happened that was put into place by some of the early followers. And at that it isn't even the earliest interpretation! The earliest interpretation appears in the Gospel of Luke, "they supposed that they saw a spirit" (Luke 24:37). Now the Lukan author is going to make an argument against this interpretation, but this argument is later than the original holdings of the disciples. It is a corrective to an earlier tradition that Peter and Mary had visions of Jesus as a spirit (or ghost?!) after his death.
We don't have to look hard to find all sorts of psychological, anthropological, and sociological studies to point out that the death of loved ones, especially traumatic deaths, frequently result in post-traumatic experiences including vivid dreams and sightings of the deceased. In fact, I can relate to this very well on a personal level. When my mother died unexpectedly ten years ago, I experienced very vivid dreams of her visiting me. In these dreams, it was as if she never died, she had only been hidden away by the doctors, who continued to work to heal her. Once cured, she would walk out of a door and embrace me. I would respond stupefied. Why would the doctors have told me she died, when she lived and they knew it? Always there was a sense of relief that she was really alive.
If I had lived in a society that understood dreams to be messages from God, visions to be interpreted, I might have understood my own dreams of my mother as a religious experience, rather than as one of the ways that my own psyche was trying to deal with and accept her death. Given what the gospel narratives tell us, the visions of Peter and Mary (and others?) were interpreted as religious experiences. The simple explanation that they saw Jesus' spirit appears to have not been enough of an explanation. It wasn't simply a ghost. They move to locate their visions of the deceased Jesus within their Jewish belief system, to align them with Judaism's teachings about what happens to a person after death. This is how and why the visions of Jesus' spirit begin to be perceived as visions of Jesus resurrected.
The resurrected body was understood to be a different thing by different Jews. There was no consensus teaching. There appears to have been a wide range of belief even among the first Christians, from the belief that your raising will be as a new spiritual body of glory like the angels (Paul) to the belief that your raising will be of your physical fleshly body from the grave, a body that still needed to eat (Luke). The empty tomb stories were created in order to correct the earlier tradition that Peter (and others?) had visions of Jesus as a spirit, and its original interpretation (which Paul knows and supports, and Luke alludes to), that Jesus' resurrection was a resurrection of Jesus as a spiritual body.