For sure that ditch behavior could use improvements, it's just technically difficult. I'm guessing of course, but it's a somewhat educated guess.
For a rough description, TFP doesn't want zeds to dig terrain for pathing "in general". For example, you can make a steel wall on sand, and have the zeds beat on the steel, not dig under it for the cheaper path. This looks and feels better in gameplay, otherwise all your bases would need a two-thick floor of the same wall material, to keep that path more expensive. For this reason, terrain blocks are made somewhat "sacred", zeds don't consider them destroyable "in general".
There might also be good optimizations to be gained from considering terrain solid, but that's just implementation details; whether terrain is "actually solid" or just "stupidly expensive" for AI pathing, the results are similar.
The cases where zeds attack terrain are when they don't really have anything else they're able to do. So if you lock them into a pit, they'll chew on the walls; if you hide under terrain and they can't find another path, they'll dig to you. These are chosen specifically by the pathfinding running into a dead end, after which they circumvent the "sacredness" of dirt.
With a ditch, there's a problem of detection. The AI should somehow decide that "this terrain is different", to break the sacredness, but it can't. There's no easy way to distinguish a ditch or a dirt wall within the A* search; as long as the A* doesn't fail, the zeds are blind to the existence of the ditch.
It might be possible to detect via other methods, and then run some simulated random attempts at breaching it, before sending zeds to do just that; but those mechanics are not in the game; and I doubt there's any plan to implement any.
That aside, being a bit of a ditch connoisseur myself, I don't think they're that exploitative; they're still rather fickle. You can't isolate yourself from the zeds with them, you can only re-direct their pathing; if you leave no path to you, they'll happily eat a dirt wall. Even the muppet suffered from that in his latest d7000-horde video..
Re-directing pathing seems to be the main current meta, most people tend to build elevated bases with a single point of zombie entry; I don't consider that exploitative. And for a "minecraft tower defense", I think it's a prerequisite - methods may of course vary in "fairness".
Basically the same applies to the force fields, they're not really that exploitative.