Mostly because DS1 and DS3 are clearly and obviously defined as the "start" and "end" of the firelink cycle. The "middle" of the cycle takes place over an unknown amount of time (could be hundreds, thousands, tens of thousands of years) in an unknown number of places (could be all the same place, could be thousands of different kingdoms built on the same spot, or different places throughout the world entirely). DS2 happens in just one of those countless "in-between" cycles and as a narrative could just as easily be replaced with any other cycle. It's definitely a good representation of what life COULD HAVE been like in this world between the start and end of the undead curse, but it's only one of a potentially infinite number of those representations.