People keep buying it, even at times full price. So why bother releasing GTAVI? GTAV also has no real competitor.
It is like World of Warcraft, keep releasing small content packs and people will keep playing, even re-release the old version of the game.
When GTA V is 15 or 20 years, I bet they release it again.
New releases can be risky, just look at Nintendo.
Wii was selling well and kept selling well, but then Nintendo was like nah lets move on to the Wii U instead.