Every day we deal with a thousand 'games' that are really uninteresting because the payoff matrix is so lopsided that we don't even think about it.
If we take those situations out of the denominator, what does the fraction look like?
In the context of evolutionary psychology, taking those situations out of the denominator might be a bad idea, leading to biased thinking.
Hannes Rusch argues that the Prisoner's Dilemma is best understood as merely one game of very many:
http://www2.units.it/etica/2013_2/RUSCH.pdf