For the same reason that Super Mario Bros. was sold by itself...
What that reason is, I don't know for sure. A few guesses could be for if the game breaks, if the player wants a full-on case/manual with the game (e.g., collectors), or for when they stop bundling it. I think the last reason is the most probable. They aren't going to bundle Wii Sports forever, so they need to give future buyers options. If they released it later, then Wii Sports would be a "new release" months after it was first availible.
Side note: When a game is bundled with a system, does it count as an official "sale"? I have heard that the U.S. does not have any accurate/independent game sale records, but I heard that Japan does. Will every Wii sale count as a Wii Sports sale?