Implementing cover would also require a change to their ray-tracing/cover detection/"am I able to hit" algorithm. Currently, if you can draw a line (or arc) to any part of the enemy's body, you can hit them. This doesn't work with cover. You need to draw at least 4(?) lines to different parts of enemies, probably more.
Not arguing against Larian doing it, but it's not nearly as simple as Solasta/XCOM's grid-based cover system.