Not that you likely care, but the reason why they went with the whole “ciphers” thing is to give Luke a reason for being unable to read the entirety of the Jedi texts himself. He DOES in fact read as much of them as possible, as he is able to. He jots down the decoded text for reference, and these are his “notes”. The deleted caretaker village sequence, which is canon, confirms that he read at least some of it. This is why Yoda jokes about them not being page-turners, because it was an extremely complicated process to decode them. And why Luke doesn’t give him an excuse like “I tried to read them” because we already know how Yoda would have responded to that.

EDIT: As for why the ancient Jedi coded their texts in the first place? Maybe they were just overly protective of their secrets. Or maybe the Sith were at large at that time (no rule of two) and they needed to protect them.