I blame the Ellimist. Yeah, I hate it when everything is explained by "the Ellimist did it", but there's no better explanation here.
#1: Two of the Animorphs (Jake and Rachel) have siblings that could stumble across the diaries and wreck everything. Tom is a Controller, so that would make his discovery a million times worse. There's no way the Anis would be taking that kind of risk.
#2: There is no way on this green Earth that Marco, highly-paranoid Marco, would be writing this stuff down unless Visser 3 was holding a Dracon to his head. Even then I doubt it.
#3: As others have said, the Animorphs had their memories wiped after various missions, and they hardly had time to write this stuff during those missions.
#4: Rachel died, yet her writing continued while she talked with the Ellimist. That means either Rachel (and the rest of the Animorphs) were writing the books or the Ellimist was, and Rachel wasn't in any position to be writing. Even if she had been, it would have been the last thing on her mind.
#5: We know the Ellimist wrote at least one of the books (the Ellimist Chronicles); why couldn't he have written all of them?
The only flaw here is in #4. Rachel's writing continued until she died, not a moment longer. The Ellimist should have been able to keep going after her death, while Rachel obvious couldn't have.
I think there's is a decent explanation, though. As we discovered in #7, the Ellimist doesn't know everything. Perhaps he could only access the Animorphs' thoughts and memories once they had been created? Since Rachel died mid-thought, he would be unable to finish the sentence for her.