Yeah and I couldn't tell you how many times I was climbing up a mountain and reached a point where I could no longer pass and I was just like, "Damn it! I want to get to the other side!"RDR did it what are you talking about?
The obvious reason for GTA always being islands is that of a hardware limitation that likely won't be overcome for decades. If you surround a body of land with water, it's more believable than a body of land surrounded by impassable, un climbable mountains of imprisonment. Honestly I don't know why they even have you break down after going so far out. Someone correct me if I'm wrong but in previous games you could go out as far as you wanted correct?
However the difference in RDR, in my opinion, is that your means of transportation is almost always slow as snails compared to that of modern day transportation, thus making the RDR map feel big by itself. After all the exploring it's not a huge deal that by the time you reach a wall you can't pass.
With the jets and sports cars of GTA I feel like those impassable mountains would just leave me with a claustrophobic feeling. Not to mention you couldn't even swim in RDR which I didn't get. It's all a psychological thing but to me, using a water boundary gives me more closure that the actual map has ended as opposed to a hard boundary such as walls, buildings, trees, tall mountains, and road blocks. The latter I'm just left feeling like I wish I could get to the other side.