why they keep releasing and we keep buying
It could also be because of those cool looking weapons they design for each character in the game even though all of them basically work the same. Different characters with different weapons with what looks like different sets of moves might be what's appealing to gamers.
Also don't forget the large maps which players run around in can also be a cause. Maybe running from one point to another on a large map while killing hundreds of soldiers is a rather exciting thing to do. Supposedly familiar looking landscapes that try to replicate ancient lands may be eye catching to some that they just want to see it over and over again even though they know that they are all the same.
Perhaps the ultimate reason is because gamers finally get to be an ancient legend from the warring times and play ruler on a map that is ancient Japan.
In the end no matter what the reason may be, one thing is guaranteed; that no matter how many sequels they release fans will still play it despite it being the same thing. For a fan who owns every Dynasty and Samurai Warriors game, he or she will know the reason. For a franchise that has lived through so many sequels, the mere line between good and bad can't be used anymore. This game should satiate the appetite of fans until Musou Orochi comes to the US.