I'm amazed by how many people fail to comprehend how replays work.
Notice how you can switch between 874 different views during a replay, rather than being stuck with just one? Notice how you can pause it and jump straight into the replay to take photos, with full freedom to move about in the replays? Put in your copy of your favorite movie and try doing this. You can't, because your movie is just an as-is video, while game replays are live, real-time renderings, just like when you played the race.
The replays aren't videos. They're sets of instructions that tell your Xbox 360 what game data it needs to load and what exactly to do with that information to render the replay in real time. This means that, in order to play the replay on a PC or laptop, it must have an Xbox 360 emulator to run the game and all of the content of the FM4 game discs to pull from.