After exhaustive, and at times confounding, research, the Grill Party investigative team has reached what it believes the reason for non-use of turn signals. It all comes down to politics. Of course, what else could it be in this day and age?
Left-leaning drivers can only use their turn signal when turning left. Right-wing drivers can only use their turn signals when turning right. This hard line stance not only shows to the world the political orientation of the driver but also shows the entrenched inability to even consider an alternative viewpoint. Public safety is completely irrelevant when one's staunchly ingrained political orientation is at stake.
Therefore, at any given time, approximately half of the drivers are using their turn signal only half of the time. Voila! Non-use of turn signals explained.
As always, slow down and grill.