I believe it’s also partly because encounters couldn’t change/replace terrain at the time that encounter was made, only remove (mine) it. So I suspect the full story/answer is
“as a design choice they wanted the goblin camps to have a dirt floor, at the time the encounter was added this was the best way to achieve that, and once new options were added there was too much higher-priority work being done for them to justify going back and changing the encounter; particularly when they’d already put the effort into a work-around to replace the grass”
I’ve put that in quotes though because it’s just my interpretation, though.
One interesting thing to note: if using a geomancer to replace the grass, all of the grass will be the default level colour; so if the camp intersects with any of the lighter stripes around the edges of level changes then the geomancer’s grass won’t match with them. However, the “un-mine” function of replacing the camp’s grass will restore the blocks as they were before the camp spawned (or at least that’s how it went last time I bothered to fill the hole – often I leave it and some of the walls to mark the site of the battle, or build over it, or do some other roleplay-driven thing.) So the dirt pile is clearly remembering the terrain that was there before, rather than just creating new terrain… actually if I recall correctly the way the grass is replaced is the same method that building demolition uses to restore the terrain, I know the dirt pile was added at the same time as building demolition was.