This does hint at the root of the matter: letter writing is a format dictated by the constraints of communication. These constraints changed between 1815 and 1915, and are all but gone today - so the format of the communication changes accordingly.

The letters of yore we revere today were written by intellectuals and are fascinating, but that wasn't what all letters were like. The letters written by regular people were as banal as the IM threads exchanged by regular people today. Intellectuals still produce long form material, just not necessarily in letter form.

This same concept, by the way, is why we always say that music from $previous generation is better than music from $current generation. We have access to all of the bad music from today, but the only music we remember from the good old days is the good stuff, this is doubly true if we weren't alive back then. We only have the music that survived because it was good (popular and lasting).

I can never remember exactly how a block letter is even formatted because I write them so infrequently.