This has been a topic talked about since 2.0, but it won't change. When it was mentioned the devs gave us the final boss in Stone Vigil HM who has a random agro table. So the devs clearly didn't understand what the players meant by more random encounters.

But the hard truth to why we won't ever see anything off script is efficiency. Its easier to code a fight that follows a script of timed abilities, its harder and more prone to bugs to develop a legitimate AI that will be more reactive or random with its abilities. In fact think back to every encounter from bosses to random enemies, you'll notice they are almost all on timers. As a tank its noticeable when you pull a group of the same enemies. If they all agro at the same time you'll see all the same attacks go off together in unison. The aevis mobs in stone vigil NM are a prime example of this. If you agro all 3 in a pack at once, you'll get 3 aoes dropped all at once.

The simple truth is the devs either lack the ability to develop mob behavior that isn't "follow the script", or they won't do it because it would take more time and effort and won't fit the copy-paste high throughput schedule they have set up.