I think the answer is either that the passwords on the books would be so insecure they'd be pointless, or they'd be so impossible to maintain, it'd be untenable.
The books are not intended to be completely inaccessible to students; they are merely restricted to those students that obtain a teacher's permission. Any password would have to be shared with any student who obtained permission. Of course, then the password would only be as secure as the least trustworthy student to receive the password. Worse, because the password is spoken, a well meaning but careless student could easily leak the password.
The problem becomes even worse if all the books have the same password, or the passwords are rarely or never changed. Then it would be even more likely that the password would be leaked by someone. But imagine if the books had unique passwords that regularly rotated. It would quickly become impossible for anyone to manage without writing the passwords down, and written passwords can be easily stolen. And even then, there's surely enough books that managing all those passwords would be difficult.
Compare with the Maurauder's Map, which only ever had a few owners that knew the password. And even then, it was still foiled by the twins, who had discovered it and the password without being told about it. There are even multiple other incidents in the books where passwords are foiled, like when Sirius Black uses Neville's written password list to get into the Griffindor common room, and when Harry just guesses the password to Dumbledore's office.
In conclusion, it makes sense to protect the Maurauder's Map with a password since most adults wouldn't be bothered to try to figure it out, and confiscate it without bothering, saving the owners from a worse punishment, but with a library full of books, it seems simpler and just as effective to just have a librarian supervise them.