It’s his scholastic opinion, based on the anthropological and sociological evidence we have. That’s pretty strong evidence.
Crossan's view is one opinion that skeptical scholars have about what happened after Jesus' crucifixion, but there are other views. The scholar Géza Vermès, a skeptic of Jesus’ resurrection,
wrote, "[W]hen every argument has been considered and weighed, the only conclusion acceptable to the historian must be that … the women … found to their consternation, not a body, but an empty tomb."
We could argue which position is right if you want. However, since virtually all scholars agree 1) that Jesus was crucified and 2) "that Peter and the disciples had experiences after Jesus’s death in which Jesus appeared to them as the risen Christ" (Gerd Lüdemann [Germany's leading resurrection skeptic],
What Really Happened to Jesus, pg. 80, quoted online
here), will you grant these?