Congratulations and "Thank You" are not synonyms. When you congratulate me for a purchase you are saying that despite all the difficulties of making a purchase I managed to anyway. (that is either you know your purchase process sucks, or you know I'm a cheapskate who would never spend money). When you say thank you, you are acknowledging that there are a lot of different things I could buy instead, and many different vendors i could choose, but you appropriate that I choose you anyway. In almost all cases you want the later, so why does everyone say congratulations?