Because the car is moving, and your eyes can't track the movements fast enough, it makes any diplopia (double vision) worse.

I had diplopia a couple of years ago. My eye doctor took opaque scotch tape and put it on my glasses, on the glasses lens for the eye that was causing the diplopia. Covering the glasses lens with the tape, basically made a patch, so that I was only looking out with one eye. It really helped, and was a heck of a lot more comfortable than wearing an actual eye patch, which I've done before.

The tape didn't hurt the lenses of my glasses.