eh, DQ may have the same core gameplay but none of them are the same. they each have something that makes it different from others in the series.
DQ1 - first DQ, one character battles, revolutionary back then.
DQ2 - introduced a party mechanic for battles, much bigger world.
DQ3 - first DQ with a job/class system, you could create your own party, you can choose between female/male hero.
DQ4 - the story divided in chapters, you follow each character throughout their own chapters till they eventually all join together. introduced the wagon and for the first time ever in DQ you could switch characters around. could choose between female/male hero like DQ3
DQ5 - very unique scenario in which you follow the Hero's life from childhood to adulthood, first DQ to allow monster recruiting.
DQ6 - introduced a new job system to the series, a very unique scenario involving 2 different worlds.
DQ7 - features the most unique and interesting scenario in the series, that involved time traveling to unlock new islands/continents. the longest DQ game ever. it expanded on DQ6 job system.
DQ8 - first DQ to be fully 3d, took a more cinematic approach for storytelling. first DQ to have voice acting too.
sure the battle system may not be drastically different between each entry in the series like FF tends to do, but that doesn't mean every DQ game is the same.
the series innovates in its own way, it innovates by presenting very interesting scenarios that directly affect gameplay. innovation is not changing something just for the sake of changing it to be different.