The games industry has had a massive problem from day one, one that nobody is quite sure how to fix. This problem is the reason thousands of arcades had to close down after the 80s, and it's why Atari, Sega, NEC and countless other electronics giants had to bail out of the console business after losing millions. It's the reason why even Microsoft has lost billions on its gaming division.
The problem is that video game players simply won't keep playing without a new gimmick every five years or so. Where people have been happy watching celluloid movies for like 80 straight years, for whatever reason gamers won't keep playing games unless given a completely new format every half decade.