I've been reading a book by fantasy author Terry Brooks called 'Sometimes the Magic Works'. He relates a series of useful hints for authors based on his own experiences, which include writing the tie-in book of the film for 'Hook', and later 'The Phantom Menace'. His experience was that the writing of a book is absolutely nothing like the process of making a movie, and that his expectations as a writer were hopelessly wrong when applied to movie-making. Yet fundamentally, they are both forms of passive, linear, closed-ended entertainment, quite similar when looked at from the outside.
So if those forms can be so different, how can anybody expect to equate movies to games - which are active, and often non-linear and open-ended - and expect to be taken seriously?
I was tempted to say that its a specialisation of the "people are stupid" theory, however a moments more thought gave me something different.
The problem is that, truely, very few games are as you say. Most games have a static story line you follow. Infact, looking at my shelf I can see FPS, RTS and RPG games, all of which have a mission or objectives based system which means you often HAVE todo things in a certain order.
RTS games present you with missions, its rare for a game to let a campain be played in any order you wish.
FPS games generally have a specific goal and the story is split into chapters (HL and HL2 for example), but all the time you are driven down a linear story.
RPGs are one of the few games which aren't totally linear, you often have side quests which can be completed out of order, however there is still a singlar driving story line at their heart.
Looked at in such a manner, I find it easy to see why someone on the outside would view games in much the same way as a film, the only difference being the user interaction is active instead of passive. Both have a driving story line, generally linear and many games (even with sequals) have a story line which can be considered 'closed' at the end of the game.
Infact, the only two games which springs to mind which didn't have a closed story are Dawn Of War and Half-Life, both of which left a large openning for a sequal.