I would love to know your good authority, but actually the truth is that many comune records have yet to be digitalized. It is a slow process. Even NARA records have not been computerized; one needs to search through microfiche files--rather archaic in today's digital age. Or is Italy more technologically advanced than the US?
Certainly any Italian computerized records can be made available to the consulate, but if they have all these records at their fingertips or on their computer screens, why was it necessary for me to provide my grandfather's Italian birth certificate when I applied. I'm certain you had to provide your Italian documents as well. Couldn't they just look it up? Guess what. They don't have them.
Even if I accept that you are correct that ALL comune records are housed in a grand database accessible to all consulates, their omniscience ends as soon as the immigrant leaves Italy. No, jennebet, Italian consulates have no information about the immigrant after he leaves Italy. Those are on US records, records that we also provide when we apply. We bring to them information that they do not possess to demonstrate a claim of Italian citizenship. The consulate then uses that documention to trace a citizenship line. That is the extent of their responsibility.
In fact, during the major Italian migration, Italy was a very young country, and records were as poorly kept there as they were poorly kept in the US. Once the immigrant left, Italy lost track of them. This is not unusual; nor is it unexpected. In reality, the major portion of documentation required for citizenship comes from non-Italian sources.