I don't get a wave back. The times I can watch the other driver, I believe the other driver just doesn't see me. Sometimes on the freeway I can pull along side the other car and the driver doesn't even glance over.
It happens to me too. I'll be moving along and out of the corner of my eye catch a Porsche at a side street, a light, or coming the other way, and I just didn't see the other car in time to wave.
Sincerely,
MarcW.