That is not what I asked.
I asked:
Why does something's age necessarily make it more difficult to ascertain its reliability?
You seem to be under the impression that a historical account's context being old necessarily excludes it from being reliable. But clearly historiographical methodology makes use of a variety of different means in determining the reliability of an account, not just its context or age of said context. Having a recent context is not a necessary condition for determining an account's reliability, it is a sufficient condition. In the event that a historical document is old i.e. the New Testament writings, historians are able to offset any potential diminishment of reliability due to the age of the text by looking at other indicators of reliability. If these other indicators of reliability offset the potential diminishment due to the age of the context, then the account is deemed reliable by virtue of the overriding strength of the various other indicators of reliability utilized in the process.
No, I mean it practically always does, if you want to demonstrate I am wrong go into details and don't make some overarching argument that it could be very well sourced even if old (that is true). For instance we often get better documents from the roman civilization than from many of the dark age communities because they simply kept better records and had a better set of historians.
If you want to argue specifics, for things like the Gospels, we have no proper authorship, we know next to nothing about the people who authored them (they are 'attributed' to people we know very little about), and thus we have no ability to assess their reasoning for writing them or their source material.
Such things would not be missing on a modern account.
Bringing up the number of historians is pertinent only if multiple attestation is a necessary condition. But once again there are a variety of different indicators of reliability historiographers make use of. Any lack of attestation is often offset by the strength of other indicators of reliability. So this is a sufficient condition, not a necessary condition and that is if I grant that there are fewer historians as you suggest.
The number and quality of historians is a known factor in how well an older document will hold up. You need credible original sources and others to cross reference and add the context.
Historians generally get great leeway in the most ancient world because so little survives, but this doesn't mean that we aren't aware of the failing.
As far as historians being less credible, this seems to me to be something that would need to be substantiated.
Just go look over the historical record if you like. The further back you go the less professional and the more "literary" your history gets.
If you go back and read Herodotous for example, much of what we get from him is not great in terms of reliability.
Give me an example of it being accomplished and I will retract the point.
Feel free to do so at your leisure.