As for why these types of conversations frequently end up with people talking past each other, there are too many vested interests in too many different sides of the debate for people to know in advance which points and interpretations they should be emphasizing. Are we discussing history? Religion as a concept? Religion as an identity? Epistemology?
As for myself, I used to be devoutly religious, I read the KJV OT cover to cover once and NT multiple times, plus the other books of my former faith multiple times, plus a number of apologetic essays. I was also learning about science and empiricism, and eventually found that my faith was full of contradictions and grasping at ever more tenuous archeological straws, while science provided modern evidence for its claims that could be verified and even rediscovered by anyone with the time and equipment.
So in these kinds of discussions, if you happen to be talking to a person with a background like mine, if you want to go beyond just factual discussions of historical discoveries and into the loaded territory of religious historicity, you must first convince such a person that fragments of ancient manuscripts should bear any modern relevance whatsoever in the face of what they would see as the significantly more reliable epistemological foundations of modern empiricism.
No, they're not. They're hagiographies. They're not intended to be records of historical events, but narrative prose with a specific audience, intended to bring them to Christ. The only exception might be Luke, who purports to recount eyewitness reports, but even he is not intent on detailing history, but imparting meaning to an existing narrative. (This is, by the way, widely accepted mainstream theology)
If these are our best sources (and historians tend to agree that they are), anyone honest about history has to recognize that the historiography of Jesus' life is problematic, and the quantity of manuscript copies in circulation will not change that.
Whether they are hagiographies or not is for you to judge. Personally, the gospels more often than not present their subjects in a difficult light. For example, the Messiah (King) is to be crucified. This would have been anathema and foolishness to most Jews and Greeks of the day.
One does not consider a document to be historical merely because it claims to be, or exclude a document merely because it fails to do so (as you require of the gospels other than Luke). Any and all documents available should be considered and weighed appropriately. And in fact, incidental evidence should be given all the more weight.
I'm not doing the judging; academics are. The word I used may be imprecise or even anachronistic, but the thrust is the same, and validated by academic judgement of the texts. Genre criticism is an aspect of theology, and it crosses over with historiography. In the evaluation of practitioners in both disciplines, these are works that are not intended as historical accounts.
While, as you say, this does not invalidate them as historical sources, it doesn't mean that every event recounted and every argument made therein is one that can be considered a valid historical occurrence. Many of them aren't even approachable by the historical method.
Here are some examples then:
"It was definitely written in the first few centuries BC."
Firstly, the New Testament documents were not written "BC" but AD. Secondly, most scholars will agree without question they were written within the first century, and no later.
"22-25 years is still a very long time before any written records emerge"
22 years is actually reasonable for most history, and these documents are ancient history. Further, there is evidence to suggest that Matthew and Luke for example had access to even earlier sources. In any event, this kind of historical distance is still short enough for any inaccuracies in the accounts to be corrected, provided the eye witnesses are still alive, which in this case they were. Remarkably, 1 Corinthians 15 recognises this and is careful to mention that "most of [the eye witnesses] are still alive, though some have died".
"That doesn't make it historically accurate, it just means lots of people read the stories."
Actually, having lots of manuscript copies (in different languages and from different locations and places) is the key to textual criticism, which was the context. Having thousands of manuscript copies, as in the case of the New Testament documents, is mind-blowing when you compare it to the typically handful of copies for ancient history of the same period. As an unbiased person looking into history, you really couldn't ask for better.