Eye tracking and depth of field adjustments are the missing pieces. A HUD has to be able to stay in the same focal range as what is viewed beyond it so you don't lose concentration on the task at hand.

In fighter jets, they project onto the visor. Obviously not the most convenient method for an automobile. There have been attemtps to figure out depth of field but it's tough. https://www.sciencedirect.com/science/article/abs/pii/S00304...