Because American have a very poor culinary tradition (it's improving) so they can't think of plates prepared with different "principles". Most cuisines around the world use a lot less meat if they use it at all, and manage to do that while being delicious.

Italian food is a perfect example, it's so easy to be vegetarian eating Italian (the original version, not the one covered in meat and cheese you find in the US).

Same goes for almost every other culinary tradition in the world, meat used to be expensive.