If there were only a few seconds between the GC leaders and those behind him, I'd be sure the last stage will be a race for GC also. But then you are also going to have to fight the teams that are putting their sprinters up to win the stage.
On a stage like the final will be, getting more than a second or two on the lead GC will be hard to do. So I think that's why it's mostly a presumed win and more ceremonial for the GC.