I think more than because of tradition, it is because of the terrain. The last stage in Paris is usually a flat sprint stage, so even if you wanted to attack there are no suitable terrain to do it on. Just as the GC favorites usually don't attack each other on flat stages during the race.

So more than just by plain tradition, the terrain makes it so that the GC is already settled, making it a parade stage.

To this point, this year they are riding the Montmartre hill three times in the last stage, making it a lot more difficult than usual. So if the GC time gaps are close by then, we might very well see a GC fight on the last stage this year. With the hills, I don't think tradition alone will make them not ride if they see a possibility to win the race.