I'm pretty sure turn based combat originated from MUDs/command-line RPGs where the action was done in a very step-by-step, linear way. And there are various reasons for that, but the point being I suppose it stuck around.
I don't find it very immersive either and, to me, it's fairly outdated. I find (as you pointed out OP) games such as Oblivion, Dark Messiah Of Might and Magic, and Dragon Age much more immersive and involving. They feel very visceral (especially Oblivion and Dark Messiah) because of the direct control over characters and their attacks.
The reason it is still around? Perhaps it may be because it's easy to manage, easy to code, more involving for the more "mathematical" gamers, and effectively progresses the game.