Simple question. The answer might be simple for some and not so simple for others, but with the discussions going about everything from supposed lack of content to Rockstar's greed to every point in between, you who read this chances are probably still pops the disc into your console (or if you're like me and bought it digitally, just starts it up) and plays the game.
I guess the OP should answer first so...
I play the game still because despite a rocky launch and despite technical setbacks with bugs and glitches and all that still holds the game down, it's fun to play. GTA was always open ended, but it's games have been linear still in that the story has it's natural ending. GTAO is never-ending, and the more that's added to it, by either R*'s making or my own, the more dynamic it becomes compared to single player GTA. There are big plans for this, and plan on being around to see it all happen, no matter how long it takes for it to happen.