An interesting question, no? I must admit, I have quickly browsed an open diary, many years ago. I probably wouldn’t now, unless reading it can help me to help the owner or further understand some of their life choices.
A person’s diary is their personal right to privacy. No one has the right to look into something so personal, unless it’s your child and you are fearing for their safety or mental health, not because you just want to be nosy. So no I would not intentionally look at a person’s diary left open.