https://www.gamespot.com/articles/gta-5-story-dlc-expansions-wont-happen-and-heres-w/1100-6454293/

Rockstar Games never promised it would release single-player expansions for Grand Theft Auto V, but some fans hoped the developer would. After all, Grand Theft Auto IV had some excellent story add-ons in the form of The Ballad of Gay Tony and The Lost And Damned. Rockstar has instead focused on updates for GTA Online, which have been plentiful and substantial. Now, Rockstar design director Imran Sarwar has explained that it was "not really a conscious decision" to not release single-player expansions for GTA V. Instead, it "just happened" to work out that way, he told Game Informer.

Sarwar added that Rockstar "would love to" make single-player DLC for its other games in the future. "As a company we love single-player more than anything, and believe in it absolutely--for storytelling and a sense of immersion in a world, multiplayer games don't rival single-player games. With GTA V, the single-player game was absolutely massive and very, very complete," Sarwar said.