^It is because Co op is a minimum of 2 players, meaning the story has and the map has to function with two players if the third player is unavailable.
That's why you have the third player saying unimportant crap. (Chloe lol)
And with two players only, during conversations the characters have longer pauses, because there is no 3rd person to say the "unimportant crap"
"It seems to be a part of a carved stone tablet"
"Well, let's find the rest of it."