If you've been paying attention, than you'll have noticed that a lot of games lately that offer Co-Op experiences are lacking in the Split-Screen Department. Well, as you probably guessed, the reason for this is money.
Publishers are telling the Developers to exclude Split-Screen Gameplay ("Couch Co-Op" as I like to call it) from their games if they have Co-Op. The reason for this is, again as usual, very stupid. They believe that by doing so it will force Gamers to purchase an additional copy to play with their friends, thus increasing their sales.
That explains the Consoles, but when games have Split-Screen on a Console game and they then port it to the PC and remove the Split-Screen, the reason they do this is because they believe it will curb piracy.
That is quite possibly the dumbest excuse I have ever heard, and I didn't believe it at first until I was told directly by my friend that runs a smaller development company.
Gaming is headed down even further, and I will continue repeating what I always say: BOYCOTT THESE HORRIBLE PUBLISHERS.