What is boring for some people is fun for other people. Wandering around, exploring the game world, hunting and such is what made RDR interesting for so many people. It wasn't just a western game, it was a full blown wild west world. In it's particular case, I think there is a considerable amount of people that didn't buy it for the story, but for the sandbox world. So, it doesn't matter whether they finish the game, because it doesn't mean they didn't enjoy it.
While I do believe there is a lot of unnecessary padding in games, not everything would be benefited by being cut shorter. Hell, there are even people that like grinding, so it's really something to be thought in a case by case basis.