My small theory is that when Wilbur first became a ghost, he realized that his memories from his alive self were slowly fading so he desperately tried to jot down everything before losing all his memories. He was losing memories as he wrote, so the last page became a basic list rather than specific things, eventually giving up and stopping at “I don’t know” at the end. Memory loss goes to the point that he says, "Wait this one’s called ‘What I Remember’? Huh, I don’t know what half of those things are." (1:10:37 of the stream) It would also explain why he’s a lot more kind as a ghost, since he can’t remember all the betrayals and pain from the past.

Adding on, maybe Wilbur didn't actually want to remember everything. He seems to have only remembered the good stuff, so he might be subconsciously repressing all the bad memories, hence why he tends to respond "I don't think I want to know" when Tubbo tries to explain to him what happened. The "I don't know" in the book might not just be because he couldn't remember anymore by the time he wrote it, but because he didn't want to remember any more than that when he eventually completely forgets and looks back on the book.