Modern AAA games like Call of Duty: Modern Warfare II use strict, advanced digital rights management (DRM) and online verification.
Activision Blizzard uses a sophisticated entitlement system. When you buy Modern Warfare II on Battle.net, the key is generated, hashed, and stored server-side. There is no master .txt file floating in the wild.
Sometimes, game publishers or retailers provide activation keys in .txt files for users who purchase the game through specific promotions or from them directly. These are usually sent via email or made available through a user's account on the retailer's website.