Plus you get weeks and weeks of (presumably good) holiday sales. Halo 5 as an example may be off to an underwhelming start, but it still has several weeks of lucrative selling ahead before Christmas. Same to all other games launched in October/November.
Halo 3 sold 8.1 million from its release to the end of the holiday period while selling 3.3 million over its first 12 days. So the holidays can definitely provide a boost.