Could be any number of reasons.

They don't know the message books exist or that they're actually functional instead of just decorative.
They hit their item limit and they wanted something else more than the wanted the message book.
They think the message books are ugly.
It doesn't fit in with how they've decorated.
They don't care what other players think about their house.
They don't want to deal with rude or advertising messages that some players enjoy leaving (my house does not exist to advertise someone else's ERP club on a different world).

Why get so upset that there isn't one? It's their house, not yours. Be happy they're choosing to share it with others where many don't.

Quote Originally Posted by ERMITANYO View Post
Possibly in future housing iterations that are instanced, a log book may become a built-in housing feature.
I hope not. I don't want the game deciding where the message book is going to be, or even that I must have one. I don't always use it when I redecorate.

The current built in "features" in housing decor are bad enough. Let's not add more than have the potential to be annoying depending on how we're trying to decorate.