SK will also give retroactive credit for flights flown 30 days prior to enrolling. However, I do not see why more airlines won't give retroactive credit as far back as a year or more. The closer a frequent flyer is to an award ticket, the stronger his/her loyalty to that airline will be. Most of my miles are with *A but I also have a few thousand miles with AA and Delta. When finding a flight in the US I would always look at United first before exploring other options. It should be in the airlines own interest to get you "hooked"
on its FFP and the best way to do this is to give you credit for your past flights