They can, it's just a question of priorities and resource management. Spending time on making sure AI plays nice with ladders is time that can be spent elsewhere so they figure they can leave it by the wayside. No enemies in Divinity 2 can climb ladders. Your pet just immediately teleports up. Alpha Protocol is an example of a mismanaged game where all the awful erratic ladder behavior was left in.Bethesda also can't code climbable ladders. Do you really, really think the engine is the limiting factor?
See when you say cheap potshot things like this it leads me to believe you're a charlatan.
I hate Bethesda, not because they create bad games or rape my favorite franchises (well partly for that too), but they are the only AAA RPG developer who have no money or time restrictions on developing their games. Yet they are completely satisfied creating shitty games because they know no matter how bad they are their fans will buy and praise them and reviewers will give them perfect scores. Imagine what could have Troika or Obsidian accomplish with those kind or resources, I bet one of those things would be ability to climb ladders.