Larian aims for systemic consistency, so if they were to add swimming, they would have to add swimming everywhere.
Personally, I don't see a point to it. We have shallow water, which is sort of like swimming, but still allows for gameplay but with debuffs. Swimming would either require an entirely new set of gameplay systems, or would be pointless (you can move through it, but not do anything interesting). As such, deep water acting as a "invisible wall" serves game better.
