Because it's not the same thing? You're running the game and playing with other people...your friends. And while it's cooperative (at least, it's supposed to be...), people are unpredictable, so the players will always be thinking of something clever that you hadn't considered, or be pitifully clueless and miss an obvious solution and thus make things much harder on themselves than necessary. It's fun in its own way. Not sure how else to explain it.
And that's just the story / role playing side. When it comes to combat...it varies by DM. But for me...I genuinely don't want the player characters to die, then they might switch for a new character and all the story built up around the prior character is gone. At the same time, nothing is worse than players knowing they're immortal and ceasing to care about "threats." So you have to kind of walk a tight rope...taking them just to the brink of defeat and putting the fear of death in their hearts, but they come out victorious in the end. (of course, there's some wiggle room, like if they realize early they're doomed and can escape, or if I subtly start using non-ideal tactics if it seems I made the fight too hard...otherwise it'd be impossible to pull off for hundreds of encounters)
Why not try writing a book using the campaign as the background? ---