It’s concept art. I’d be willing to bet good money the original concept art on the biters and their bases was a lot more grotesque than what we actually got in game.
To me, it also furthers the dynamic of the game: on the one hand, they’re grotesque aliens, ew, kill em (you wouldn’t have that desire if they were beautiful, like the Na’vi, would you?), but on the other you are the invader and it’s your polluting of their planet that agitates them (technically, you as the player are the bad guy if you choose to pollute and attack them for their land).