because in a manga adaptation you already have the main story, plot devices and such. you just need to add more frames and animate the events that are already there.
a game almost NEVER is the same. have you played the original kanon or shuffle games? other than the main plot being the same, everything else you see is anime filler.
Shuffle for example, since its a newer game. there were no clubs. there were no crazy events happening all over the place. kaede did not have a crazy obsession over rin (Not like in the anime at least) and she did not detest Asa to the point of death.
as for kanon, each girl had their own paths. the anime focused only on key points.
now shuffle and kanon 2006 were good animes. but shuffle memories was not. please do not associate shuffle with shuffle memories. one was a good anime series, the other was corporate cash-in.
as for manga-
hayate no gokuto follows the manga pretty much precisely from what I hear. not in order, mind you. if the manga is good, it will be really hard for the production team to mess up since they are just adding color and voices. but if the manga was bad in the first place, the anime will be bad (and vexx would like me to point out that koi koi 7 is bad right now
).
the problem with game adaptations is that alot of studios are given leeway, too much in my opinion. that is why anime producers become famous or infamous. Look at negima, all the problems were blamed on the production crew for not following the manga, right? no one points out that ken akumatsu actually made a crappy manga. it is because the production team took too much leeway so it became known as 'their' adaptation.
you would think its not important to follow the game story, since any anime can be fine on its own, and I would agree except, then what is the point of using the game name in the first place? if you could make your own good anime without needing to rely on someone elses story it would be your name on the title, not Minori's. the problem is video games have a BLATANT history of cheap *** cash ins.
did u go see the resident evil movie? how about the awesome street fighter II movie with jean claude van dame? normally I would be optimistic, but in cases involving 'video games' which I have been playing for over 20 years now, regrettably when someone says I'm making an anime or a movie about a video game, its common place to expect it to be the frisbee type. I have played ef some but that means nothing since most of the game can be covered in 4 episodes or so. its what else they put in that will make or break a good dating sim anime series.