Maybe because it's still a lot longer than other full price games and being a karma based open world game has so much replay value and exploring opportunities.
Are you implying there are morons out there who play open world games just for the story?
Second Son doesn't offer that much replay value, you beat the game twice, that's pretty much it. The side missions also don't offer much of an experience. On the other hand, Ground Zeores offers a ton of replay value, and people still bitched about it being short.