a thief knows that a diary can be used to blackmail someone. if it has really sensitive personal information or just information the person doesn't want anyone to know. that's why a diary is a liability just as much as a bitcoin private key on a piece of paper is a liability if anyone finds it.
It is so much easier to hide a tiny piece of paper at home than it is to hide anything else. For example I have a lot of books at home, a bunch of documents, etc. that makes hiding some text inside one of them very easy.
Not to mention that a thief coming to your home would steal other valuables such as jewelry not search for your paper wallet which could be encrypted which would make it useless to them.