length is only one element of a game
that said, it's often important, because some games, even if awesome, are not worth $60, because they are too short (though there are examples of very short games that imo were worth $60 anyway -metal gear solid for example, though that game had a ton of replay value so i got far more than a single 6 hour playthrough out of it)
can you imagine the outrage of people paying full price for a movie ticket to see say, The Avengers 2 and the movie only being 16 minutes long? people would riot. it's an extreme example, but it merely is to demonstrate the point that length of entertainment is not a trivial attribute