The "borders" are part of particular art style that is essentially an upgraded, more high tech version of cel shading. For reference of the style, look up Jet Grind Radio, Jet Set Radio Future, Breath of Fire 5, and pretty much every Dragon Ball Z game of this console generation and last.
Borderlands has a comic-book art style because the devs' families couldn't tell it apart from Call of Duty and Fallout. They were shown screenshots from the three games and asked for feedback on which looked best... but they didn't even realize there WERE three games being portrayed, thinking it was all the same game.
It was called Borderlands long before this art change. Back when Reaver was called Mordecai, Zed was called Roland, the Bruiser was called Brick, and Steele was called Mercenary Scientist Dr. Lilith Cashlin.
It's a cowboy story that takes place on a hostile planet in a contested area of the ass-end of the six galaxies... that is, in borderland territories.