Why do they share verbatim passages if not copying?

The paper you're referencing suggests a revisionary model, where Matthew is seen as revising Mark, and Luke, in turn, revises Matthew. This approach challenges the Q hypothesis and instead posits that the Gospel writers were not simply copying from each other or from a shared document, but rather building upon previous works with deeper theological insight as their understanding of Christ and His fulfillment of prophecy evolved.

In this model:

• Mark's Gospel serves as the foundational account, with Matthew revising and expanding on it, integrating new insights and teachings. Matthew's Gospel begins to articulate a more developed understanding of Christ, highlighting His fulfillment of Old Testament prophecies and emphasizing Israel’s role in the narrative.
• Luke, a later Gospel, revises and expands upon Matthew, adding further theological depth, especially by focusing on the universality of Christ’s mission and beginning the story with Adam, emphasizing the extension of God’s plan to the entire world.

This view sees the Gospels as progressive revisions, rather than independent sources or mere copies of each other, reflecting the ongoing revelation and understanding of God's plan through Christ in the early church. The differences between the Gospels are viewed as intentional variations to reflect the growing doctrinal insight of the church over time.