Blanket statement with no examples or ideas.
Let me try... potatoes are starchy. Discuss.
The narrative in the game is so bad it's not even funny.
Like OP said, the beginning starts off great, story gets set up, characters are introduced cleanly (Franklin, Micheal, Madrazo) then the bank heist happens because of all the conflict Madrazo and FnM and your hype as sh*t to see what happens next. Great!
Then Trevor comes into the picture... And for some reason, the moment you're introduced to Trevor, R* decides to throw out any notion of narrative they had, Trevor swears revenge on Micheal and now the two confront each other and what happens? The two go save Tracy from embarrassing herself on TV, then for some reason, Trevor just forgets whatever the f*ck he wanted to do to Micheal for about 94% of the whole story.
Then Micheal who starts sh*tting himself because Trevor found out he's still alive, goes to the FIB and now the ENTIRE story is f*cked, Madrazo is no longer introduced until some random event in the story that i can't remember, Didn't Micheal owe him about 2.5 MILLION dollars? I'm pretty sure that one bank heist couldn't have been enough to pay him back EVEN if they didn't split the profit, which they did between everyone who was evolved.
But that's not all, now that the FIB is introduced, you're constantly forced to doing some extremely random sh*t that serve absolutely no purpose other then the fact to remind you that Micheal is in debt to them. An example of this is the heist mission where the FIB believes terrorists are trying to create nuclear bombs or some sh*t, But it's revealed on the radio that it wasn't chemical warfare but some brand new perfume
R* at this point has no idea how they want the story to progress, no idea who the main antagonist is, and no clue on how they want to end considering the fairy tail ending they made the DeathWish option where they kill every "Bad Guy" and live happy ever after.
These enough examples for ya'? I'm sure at some point in the game you asked yourself, "Wait, why am i doing this again? and what for?"