http://en.wikipedia.org/wiki/Substitute_good
Like Carlos said games have much more of a cult following then porn. For games, there are a lot less perfect substitutes than for porn. How many substitues do you have for Call of Duty compared to say creampie movies?
In porn, if people want a creampie movie, it doesn't matter if it's a hot 10 minute creampie for free from Youporn or a movie from Allinternal. That goes for the majority, of course there's still a smaller % which insists on specific producers, models etc. That % gets smaller as more is available as more free perfect substitutes become available.