I don't see it at all. Perhaps people are concerned that their gesture will be misread as irritation with something the other driver just did, so they avoid flashing.

These days there are only three instances in which I see people flashing their lights:

1- to indicate that your high-beams are on
2- in stopped traffic, to indicate they're letting you merge into traffic in front of them from a turn or on-ramp
3- to indicate there's a speed trap ahead in the direction that person just came from

From what I've seen, the only signal to indicate a "thank you" is a quick wave of the hand.