As much work probably went into GTA V and RDR2 as went into the games from 2008-2012. They're massive, I dread to think how many man-hours are going to be put into GTA VI, they always have to one-up themselves.
Games just take a lot longer to make now, especially openworld games, but look at other developers.
Insomniac (PS3/360) - 10 Games: 3 Resistance Games, 6 Ratchet & Clank Games, Fuse.
If crossgen games count towards the oldest they release on like you do then the following developers look a little worse:
Bioware (PS3/360) - 6 Games: 3 Mass Effect Games, 3 Dragon Age Games
Bungie (PS3/360) - 4 Games: 3 Halo Games, Destiny
If we look at the Halo series irrespective of the developer you can see the releases have slowed down. The 360 got 4 new Halo FPS games, the XBO currently has 1.
Bethesda developed Oblivion, Skyrim and Fallout 3 for PS3/360 and on PS4 only Fallout 4 and 76. Skyrim released 5 years after Oblivion yet here we are 8 years after Skyrim and who knows when TES VI will come out.
Mostly the only developers releasing as many games as they used to are the almost annualised games. Far Cry, Assassins Creed, Fifa, Battlefield, Call of Duty or AA games rather than AAA.
....so its just western developers that are taking their sweet time? loll