Let's say you have a 10-lines dialogue between 2 characters. That means 10 different poses for each character, as those poses changes at every single line (just open any rpy file and take a look at the code), with each pose consisting of at least 3 elements (eyes, mouth, hands). That's 60 individual elements in that dialogue matching what it's said. Now, when you got it done, you replay the whole section and feel that this dialogue just doesn't flow. You need to trim it down which not only means deleting maybe 4 lines but also rewriting the other 6 to convey the intended message. Chances are that many of the current poses for those 6 lines won't exactly fit the new dialogue (maybe there was a shocked expression when now some rolling eyes would be a better fit). So you may need to look carefully at the new scene and change 15 or 20 elements of the 36 remaining. That's gonna take some time. And sure, they could have write the whole script and change whatever needed to be changed before start with the posing, but it's only when everything is added when you can really tell if the section works or naw.
That's just how it is with this game, and also the reason why people feel it's a polished work where everything going on the screen flows smoothly, even if they can't really point at the actual reason. And again, full disclosure from devs who quickly put a stop to those wild speculations about the release date. Even if you don't believe their reasons or excuses, no one following the official sources is being misled to believe the update is close.