There is some intuitive appeal to that argument. After all, time does wreak havoc on documents, and presumably all the documentation we have from those events necessarily existed in the time of the fathers, together with further documentation now lost.
Still, the argument is flawed. The documentation may have existed, but the individual fathers may not have had access to the documents. Documents from one part of the empire were not necessarily available throughout the empire.
Furthermore, some of the fathers very uncritically accepted others' historical accounts. In some case, such acceptance was a rational necessity: there was no way to verify every detail, and what could be readily verified seemed to be more or less accurate. Sometimes a historian was working from the account of a previous well-respected historian.
Peter Heather and John Matthews have written "The Goths of the Fourth Century," (Liverpool University Press, 1991, Volume 11 of the Translated Texts for Historians series). This work is a go-to work for understanding the Goths of the 4th century, and incorporates a wide variety of historical research into the subject, including archaeology.
The authors note this problem I've mentioned above (chapter 4, p. 97, internal citation omitted):
In adapting his predecessor's narrative, however, Sozomen compounds several errors of Socrates, notably in supposing Ulfila to have been active in Gothia in the time of Fritigern and Athanaric, and he moves from he persecution of the late 340s, as a result of which Ulfila left the Gothic territories, to that of the early 370s without any apparent awareness that different events are in question, or that Ulfila, expelled from Gothia in the first persecution, had no personal connection with the second. Further, his conception of the chronological connection between the Hunnish attack on the Goths, the settlement of the Goths in Thrace, the supposed dissension between Athanaric and Fritigern and the latter's conversion to Christianity is, to put it mildly, confused.That's Socrates the noted historian, not the much earlier Socrates, the noted philosopher.
You may recall other examples of this same kind of principle. When you read the Koran, it seems pretty clear that Mohammed was under the impression that Jesus' mother Mary was the same person as Miriam, Moses' sister. The name of the two people was the same, but - as most people familiar with the Bible know - the two were eons apart, chronologically.
Mohammed is a fairly extreme example, but he was over 1200 years closer to the time of Jesus than we are, yet was in a vastly inferior position in terms of his historical knowledge. So, when we consider modern historical research against patristic historical assertions, we should be open to the idea that modern historians often do have access to better quality resources, research materials, and methodologies than their ancient predecessors.