Uhh.... who creates these walkthroughs?
I know that most games come with a walkthrough for the first level, but this is given as more of a teaching aid.
The first level of a game nomally equips the player's mind as to the capabilties of the game. Most FPS games have a learning mode/level.
Sure, getting a walkthrough for the entire game will definately help to show you stuff, you'd probably miss along the way. But isn't a game like abook, you read the book a second time and you get a different experience.
Same with a game. You and I know that playing FF7 ten different times, you will not do the same things twice.
Some games are like you said, ridiculously hard. An example of that would be Return to Castle Wolfenstein played on it's hardest level. My question is,....what were the developers thinking?