Not quite sure how it creates a social interaction when you don’t leave your respective cars.
I have no need to reassess what a gift is. I’m clear that it should be something freely given with no expectation of reciprocation. If someone gives me a gift with any sort of obligation implied then I prefer not to receive it at all.
I’ve never come across this phenomena it so wouldn’t know much about it, but someone upthread said you are supposed to do the same for the person behind you, if that is the case then clearly it is not a gift but very definitely an unrequested obligation.
You seem to suggest as much in that both you and your wife do feel somewhat obliged to do it for the next person so I’d question just how much of a purely altruistic process it is.
The upshot of it is that if a line of 20 people all do the same thing you end up with one person very slightly poorer, one person very slightly richer and 18 smug people patting themselves on the back. A small act of kindness? possibly, getting close to negligible I think.