Fallout 3? Awesome. Capital Wasteland? Good stuff. Giant, post-apocalyptic open-world? Yum! Can't wait to get out there and explore me some of that. But what's this? There's a collapsed building blocking my path? And my objective is just behind it? And the route around is ridiculously contrived and full of danger? Why, this isn't an open-world at all, Bethesda! But by the heavens, I'm bloody well going to make it one. Because of principles!
And so you sprint at that concrete slope, and charge up it for all you're worth. And then you slide back down, because the game naively assumed that you'd see a giant collapsed building as an obstacle, and so used it 'hide' some invisible walls. Undeterred, you try a different angle. And then you start sprint-jumping between angles, stubbornly performing some sort of investigative, drunken parkour. You keep sliding down, but you know--you just know--that if you can find and navigate the correct sequence of invisible cracks in structure’s design, you'll be up and over in no time.*
See also: Walking up a sheer cliff-face in Skyrim.
*You never make it up and over, and ‘no time’ translates into ‘40 minutes’.