They do, however there's one important distinction. Usually somewhere 6-8 months out from ship they fork the codebase from mainline with the discrete intention

They do, however there's one important distinction. Usually somewhere 6-8 months out from ship they fork the codebase from mainline with the discrete intention of not ever merging back. After that point all sorts of game-specific optimizations go in that would/could break other types of games the engine could be used for. Also, if you do ever have to rebase on mainline, it's a bastard to do :).