My family continually scolded my Dad for being a stubborn LLD, but we did get him to admit that he did it because the white line at the left edge of the road helped him visually. (Scary thought).
I am firmly convinced this isn't solely an older driver issue. I believe it's an eyesight issue no matter what age the driver is. They can't see dashes. They can see a solid line.
I drive a school bus for a "living" so I get exposed to driving on all types of roadways. I also see the above with drivers with the right side of the road. They'll put the right side wheels of their vehicles more or less on the right lane solid lines for miles on end - and they'll meander back and forth across it into the shoulder every now and then.
I tend to believe that some drivers have single eye dominant eyesight which makes their depth perception terrible. They simply don't know how to center a vehicle in ANY lane.
It is quite scary to have to drive near them.