This is the the developers choice to make the game this long. I can say that it's pretty much enough for a game to be finished in 1 day. Most of the games offer 5-10 hours of gameplay when played in Easy mode. Other higher difficulty levels increase the gameplay time with 30 %. You could try Grand Theft Auto 4 and 5 for a same experience, only that the game time is vastly increased when it comes to Single Player campaign.