Unfortunately that would be extremely computationally expensive, because it would require scanning the entire map all at once on a very frequent basis, and those whole-map scans are very slow - on my save, doing a whole-map scan just on Nauvis freezes the game for almost a full second, and in order to keep up with decaying corpses it would need to do that kind of scan once every 10-20 seconds or so to check for fresh corpses as well as old ones advancing to new decay sprites, and that's just not fun to deal with when you're trying to play the game.
The way it currently works, it scans only a very small area of the map at a time - at most just one chunk, which is 32x32 tiles - so it takes time for it to work through the map and get around to scanning and clearing any given chunk. Breaking it down this way stops it from slowing or freezing the game while it does these heavy operations, and even then there's a decent chance this (or my tree healing mod which works the same way) is still the mod that take the most processing time per tick if you check your time usage.
If you wanted a lore reason it takes so long, I suppose you could think of it as the smaller worms or bugs or whatever taking a while to die off after the biters they were feeding on died, or the plants or soil bacteria taking a while to break the creep down. Ultimately though the intention is for you to just forget about the creep and notice that it's gone on some future visit to the area.
If the time it takes is really bothersome though, you could try changing a few settings:
- Switching chunk selection from random to sequential will make the time it takes to get cleaned more consistent
- You could reduce the chunk subdivisions to 1, so it gets through chunks faster (but this might affect performance)
- You could turn on the map marker so you at least know when it's going to get around to the area you want cleaned (if used in conjunction with sequential chunk selection, anyway)
