It just seems weird to say thank you when someone says they will do something for you. It makes more sense to say it when they do what they say they do. Yet in society when someone says 'I'll bring that over to your house' or something like that, we are expected to say thank you when they offer and deliver.
I guess we are saying it for two different things. We are in reality saying 'Thank you for the offer' with the assumption that they will follow through and then saying 'thank you for doing it.' But it still seems weird to me.