Part of the boredom aspect really does come down to there only being so many things you can put into these games outside of missions, and even then there are only so many variations on "go here, sheet these people" you can have. Outside of that, you're really left with racing, flying planes, gang wars, store robberies, hidden packages, and the like just because of size limitations. And they have done a few other side things beyond the obvious. GTASA gave us dating and dancing , V has a pretty nice golf simulator. I suppose 6 could have a surfing mini game or something, but all of that takes up time, memory, resources and game development not devoted to the main project. And let's face it, these games are just "let's be bad guys" games and are focused on that, the same way Star Wars games focus on Star Wars stuff and sports games focus on sports stuff. If you've played one Madden game, you've played them all and other than updated rosters no one really complains that they got bored playing the same teams over and over.
I don't want it to sound like I'm defending Rockstar,, because I'm not, but I do accept that there is a point where once you've beaten the game there just isn't much left to do and if it feels like the games are built as more on-the-rails than free roam, it's because that's how the game is built. Would I like more free roam stuff to find and do? Sure. But at the expense of the actual game itself? That's not as easy for me to answer.
