This is probably why GTA 5 hasn't had single-player DLC yet
Because GTA Online is making WAY too much money.
Take-Two Interactive recently held their earnings conference call for Q3 2016 and it revealed something that we all knew was coming, but we just chose to ignore it. Rockstar's Grand Theft Auto 5 is raking in some serious cash with GTA Online.
Net revenue was $486.8 million as compared to $954 million in last year’s third quarter, which had benefited from a more extensive release slate.This result exceeded our outlook range of $400 million to $450 million due primarily to stronger than expected revenues from Grand Theft Auto V and Grand Theft Auto Online. In addition, NBA 2K16exceeded our expectations.
According to Rockstar's parent company Take-Two, the reason GTA Online is bringing in a ton of cash was because of the amount of support the developers have given the game. Rockstar's 'sustained engagement' for GTA Online has come in the form of content drops, in-game events and new game modes.
Take-Two expects GTA Online to make even more money this year, thanks to upcoming content drops. This is more than likely why we haven't seen any single-player DLC for the game. As long as people are willing to spend real cash on in-game money for GTA Online, the Story Mode DLC that has been teased for months might not ever be released.