I find it weird that you go into RDR expecting Hitman style open ended missions. Hitman sells games because thats what they do and what they market. In fact if you just completed the main missions once the game is super short. You are intended to go back and experiment. In RDR its not billed that way because they have a narrative to tell. Its not as simple as Hitmans story where the only thing that matters is you killed the target. It doesnt matter how, what you did in between, and how you got out...only that the end result is that you completed the mission. All those little details actually matter in RDR where they do quite a bit of referencing what happened earlier in the game.
Youre really comparing apple to oranges in terms of game play.