I agree with you, but I will take a guess why I think the trackers expire. A lot of people sign up as an affiliate thinking it will be easy to make money. Lets say that these people spammed a few blogs, message boards, forums, etc. for a few months and did not generate any money. They give up and stop trying. Sometimes the links they spammed do show up in the search results and so if a new player happens to sign up via that link, 100% of the money stays with Party Poker instead of 80% to Party Poker and the remaining 20% of it sitting in an affiliates account who gave up on marketing. That is my guess.