The short answer: Gospels are written in genre of Roman biographies.
A Few Academic Voice on the Issue
Craig Keener: “Most Gospels scholars today view the Gospels as belonging to the genre of ancient biography. Both supporters and detractors now recognize this general consensus…. Arguments concerning the biographical character of the Gospels have thus come full circle: the Gospels, long viewed as biographies of some sort until the early twentieth century, now are widely viewed as biographies again.” (1)
Philip Stadter: “Philosophical biography brought out the moral character of its subjects and the relation of their teachings to their lives. Aristoxenus, a pupil of Aristotle, wrote on Pythagoras, Archytas, Socrates, and Plato; Hermippus in the third century wrote Lives of many philosophers, as well as lawgivers and other figures. Diogenes Laertius’ extant Lives of the Philosophers continues the tradition. Since such lives are usually heavy in sayings, as in Lucian’s Demonax, they may be difficult to distinguish from apophthegm collections. The Gospels also belong to this category, as does Philostratus’ novelistic Life of Apollonius of Tyana.” (2)
David Aune: “ancient readers of Greek and Latin biographies from the period of the early Roman Empire (e.g., Cornelius Nepos, Plutarch, Suetonius, and Tacitus) had the same expectation as those who read the Gospels, expecting them to preserve the gist of what their subjects had actually said and done.” (3)
Graham Stanton: “…the gospels are now widely considered to be a sub-set of the broad ancient literary genre of biographies.”(4)
Regarding Mark
R.T. France: “[f]ifty years ago we were drilled in the critical orthodoxy of the form-critical school which insisted that the gospels were not to be seen as biographies, but since then there has been a massive swing in scholarly opinion on this point, and increasingly sophisticated study of the nature of biographical writing in the ancient world has led to a general recognition that, for all the distinctiveness of its Christian content and orientation, in terms of literary form Mark’s book (and those of Matthew, Luke and John) would have seemed to an educated reader in the first century to fall into roughly the same category as the lives of famous men pioneered by Cornelius Nepos and soon to reach their most famous expression in the ‘Parallel Lives’ of Plutarch’.” (5)
Footnote
1. Craig Keener, Christobiography (Grand Rapids, Eerdmans, 2019), 41, 43.
2. Philip Stadter “Biography and History” in A Companion to Greek and Roman Historiography vol. 2 Edt. John Marincola (Oxford, Blackwell pub 2007), 528, 530.
3. Quoted in Craig Keener, Christobiography, 3.
4. Graham Stanton, Jesus and Gospel (Cambridge, Cambridge press, 2004), 192.
5. R.T. France The Gospel of Mark: a commentary on the Greek text (NIGTC, Grand Rapids, Eerdmans, 2002), 5