I think you misunderstood me, Hans. The question was why Jesus didn't write anything down. Writing is not just recording marks on a page that might just as well have been recited orally, it is composition, selection, and usually includes commentary. My best friend may record my speeches stenographically, and write a nice biographical wrapper for my collected speeches. But it is a different work than what I would have written. Even if all I wrote was the foreword to his book, it would be a different book than just my friend's. And it would be "authoritative" in a different way, too.
My point about the non-canonical "gospels" being ostensibly attributed to disciples rather than to Jesus himself was my opinion about good psychology. I don't think the preferences of the proto-orthodox bishops had anything to do with their competitors' marketing strategy.
I wouldn't have expected people to make letters on demand. As Paul and Luke showed, and you mentioned, you don't need a letter to testify that The Boss gave you a timely bit of advice, something that Mohammed figured out and put to good use. Joseph Smith caught on, too.
Speaking of whom - what did Smith pretend to find? Not vanishing golden plates written by Jesus while he was visiting America, but vanishing golden plates written by somebody else about Jesus' visit. Joe was a pro. He could have said anything he wanted about his ephemeral trove. I think he's telling us something about how this game is played.
We also know that John incorporates a pretended late discovery of written testimony from someone who appears to have been Jesus' lover. Maybe you could do it only once, but it seems to me, if John can "find" the lover's memoir, then he or anybody else could just as easily have "found" a love letter from Jesus.
You only have to do it once. Not everybody has to believe your good luck, either - look at Morton Smith and Secret Mark in our own time. I'll take what he made on that. But again, what did he "find?" A writing about the principal, not a writing by the principal, not even containing a new quote.
I'll give you a free novel plot. An Ivy League professor "finds" a letter from Clement of Alexandria which says Mark went to Alexandria after Peter died. Mark gave two things to the church there: his gospel, and a scrap of paper which Peter had given him, on which Jesus had written a note to Lazarus of Bethany, which Peter ended up with and kept as a relic. Clement transcribes what's on that scrap of paper - which is all we'd have now anyway, a transcription at best - and the paper says... well, that part you'll have to write for yourself.
Why isn't that better than the Morton Smith story? It really isn't; it's kind of blah. Because adding a voyeur to Jesus' night with Lazarus isn't more "credible," but it is, apparently, better storytelling.
Coming at it from the other direction, go to an Osho (R) site sometime and read any two pages of that Godman's bilge. It's much better to have a mouthpiece. For one thing, you get an editor at no extra charge. It's a safe bet that anybody who thinks they're God can really use an editor.