I am not sure that is absolutely true. More so of certain demographics such as single people or couples. I know few families that eat out that often.

As for why, well I think time and convenience play the largest part, especially lunch. People don't have the time to prepare lunch at home and take it with them, especially when they have a long commute and no place to keep it at work. That's why I think there is such a perception there is so much more fast food in the US (I think that is a myth by the way). More places for quick lunches. I know very, very few people who go to true fast food for dinner. Some quick casual if the place is on the hot list, but most people who go out for dinner often that I know do so to cheaper sit down restaurants.

The fact that US agriculture is not subsidized to the extent it is elsewhere, and that most grocery store packaging is geared towards family size also makes it a lot more expensive and challenging for the sing person to cook.