My guess is that Rockstar skipped PS4 gen because the console is just not powerful enough. Both PS4 and Xbox One (first model). Look at Cyberpunk 2077 and what a mess it is on those consoles. If Rockstar were to make GTA VI, it would not advance the series. Red Dead Redemption 2 got lucky because the game doesn't need many NPC, many buildings and huge maps. There is open world but it's one thing to make you believe that you're in some bandit camp with 2-10 people and another that you're in a big city with thousands of people, cars and the AI that actually works (not like in Cyberpunk). Even the populated areas in RDR2 looked kind of empty... and remember how the consoles struggled with framerate in that last location of RDR2.
PS4 and Xbox One are simply too weak and you can't make GTA VI and tell people "sorry, but we need you to buy PS4 Pro and One X".
I've also seen Last of Us 2 being brought up... guys, it's a survival horror game with post-apocalypse themes made by people who were developing Playstation exclusives for generations and probably are in touch with the designers of the console.
PS5 has more power and the very fact that we can have 60 fps in some games is already a huge proof that this generation is easier for developers. Rockstar couldn't even make GTA V work at 60 fps on PS4 despite it being a PS3 game, couldn't make a PS4 Pro patch or (!) have Red Dead 2 work at higher framerate on Pro.