To my knowledge the goal of the game is to have interactive world.
For example in this video. Dan clearly says that if you kill someone there even could be an investigation.
In other videos I remember hearing that your victims will have burials and wont just dissappear into the thin air without anyone noticing.
But decomposing is different matter. It must be said that decomposition is not actually much quick.
Only after 6 months just soft parts of a body will decompose and it is not until one year after death that bones will start to show.
The thing is, the main story of the 1st Act takes just several weeks, months and even if someone where to play the game for a game year it is not something that everyone would encounter. So it is extra work that most may not even see.
But keep in mind that if you kill some cumans in the forrest their bodies will probably get eaten by animals so body dissapearing is not such a bad option. (though armor/weapons should stay there).