Same reason you might replay a multiplayer game or re watch a movie or re read a book.
Because you enjoy it.
And that’s assuming the game is the same every time you play it. Many single player games are not the same experience every time. Much like multiplayer games aren’t the same experience every time. Heck. Static movies and books can change experience over time based on how you view it in each watching/reading. Whether it’s for pure entertainment or to analyze it. Or if your own life experiences make you re evaluate the media and what it means to you.
Let’s say for mass effect. The basic story is overall the same every time. But your class can change the gameplay. The difficulty can change how you play. The paths you choose through the story can change. How you play the earlier games may change based on what you know of the later games.
Thus it’s never quite the same experience.