Kojima explained in 2011 that he kept continuing Metal Gear because,…

In a September 2011 essay, Kojima wrote that each time a new Metal Gear came out he declared to the press that it would be the last he would make. He explained that all stories and all creators have an end, and that he wanted to bring his own story to its conclusion before he reached his.

“But it's like when Hayao Miyazaki says he is not going to do more, and then always ends up doing it. I think I am kind of in a similar situation; I have ideas other than Metal Gear, and I want to go on to make other new games, but for political, business or technology reasons, there is always a time when I have to return.”

“They can't break free, they can't quit, they can't lose. It felt like that when Metal Gear started gaining fans and I started getting fan letters.”

“And in my spare time I want to establish a new game, which I have been wanting to do for a long time. I do have strong urges to create a new title other than the Metal Gear series.”

“Every game I make, I create thinking it's really, really going to be the last game I create. So I put as much as I can in and make sure I have no regrets.”