It isn't.
The rules are laid down pretty clearly (for once) about what needs to be carried, for both USA and Europe. The personal (and aircraft/engine) logbooks are not on either of those lists.
There's no point second-guessing WHY items are or are not on the list since neither the FAA or EASA say why.
Just follow the rules as written down, I say. There's enough gold-plating to be seen from NAAs without pilots deciding to unilaterally apply even more ourselves.