I would like to see it as a user, but I think keeping it up-to-date according to your game progress could be a nightmare to program.

There's also a sense that, once that text is there in the game, you're expected to read it all to have any idea what people are talking about (and the game may not try as hard to explain it to you directly). From memory it was one of the things that made me give up on FFXIII very quickly - the sheer amount of text in the glossary vs the weird vague concepts that the characters were mentioning.