Why was Mark the first person to wrote a gospel when he wasn't there? Why did Matthew write his gospel off of Mark's gospel 15 years later, he was supposedly there? Why do they accredit books in the New testament to authors that didn't write them?

The authorship and composition of the Gospels in the New Testament are complex topics that have been subject to scholarly debate for centuries. While tradition attributes the Gospels to specific authors—Matthew, Mark, Luke, and John—modern biblical scholarship often examines the texts from a historical and literary perspective, considering factors such as authorship, sources, and compositional methods.

Regarding your specific questions:

Markan Priority: The theory of Markan priority suggests that the Gospel of Mark was the first to be written and served as a source for the authors of the Gospels of Matthew and Luke. According to this theory, both Matthew and Luke used Mark's Gospel as a literary source, along with other sources such as the hypothetical "Q" source (a theoretical collection of Jesus' sayings) and unique material (known as "M" and "L" respectively). Mark is believed to have been written around 70-75 CE, while Matthew and Luke were composed later, with Matthew likely written between 80-90 CE.

Authorship and Attribution: The traditional attributions of authorship to the Gospels—Matthew, Mark, Luke, and John—are based on early Christian tradition and attribution. However, modern scholarship recognizes that the Gospels were likely written anonymously, with the titles added later by scribes or editors. The actual authors may have been followers or associates of the apostles, drawing on oral tradition, written sources, and personal experiences to compose the texts.

Reasons for Attribution: The reasons for attributing the Gospels to specific authors, even if they were not the direct authors, are varied. Early Christian communities likely attributed the Gospels to prominent figures such as the apostles or their associates to lend authority and credibility to the texts and to connect them to eyewitness testimony and apostolic authority. Additionally, the titles may have served to differentiate and authenticate the texts within the diverse landscape of early Christian literature.

In summary, while the traditional attributions of authorship to the Gospels are based on early Christian tradition, modern scholarship approaches the texts from a critical and analytical perspective, considering factors such as literary sources, compositional methods, and historical context. The authorship and composition of the Gospels remain topics of scholarly inquiry and debate within the field of biblical studies.