Pathological distress caused by being discouraged to say "thanks" on online communities
DISCLAIMER: I do not have any official education in cognitive sciences.

Maybe problem is not in saying "thank you", for me upvoting is very close to saying thank you.
I think it is rather obvious what are benefits from discouraging "thank you", it would be hard to find real content in "thanks forest".

Perhaps people wish that receiver knows who said thank you

I Joe want Marc to know that 'I, Joe' am thankful.