The other side of the coin is that it's an easy excuse for not developing the engine (takes time and money) or design (takes expertise).
No innovation = stagnation.
I think they have developed, (they were demoing new lighting, materials, shading, etc in the engine years back) but they don't want to lose sight of what a Borderlands game is. Otherwise you might as well be complaining that a new story is old hat because the book uses the same typeface as the previous one.
It's a difficult line to walk when your game has such an identity. Make it the same as the old one, and it's just the same old stuff, make it too different, and it's no longer Borderlands. I'm sure Gearbox are aware of this and are conscious of trying to push the game without taking it away from the Borderlands identity.