It's just the nature of the beast when making a game as vast and detailed aa rdr2.
They have also been busy making a current gen GTA5 and making content for gta online.
Yeah, not sure how you can look at GTA 3/VC/SA and GTA V and then essentially say "Why can't they make 3 GTA V's in one generation?"
The scale and detail required for the modern games is just so massive compared to the older ones.