Soccer is a lot more physically demanding than you think. I played in sunday leagues for about 8 years and after the game you just feel drained. Some roles such as midfielders have to cover so much ground with and without the ball. Controlling the ball effectively also requires skill, and you need intelligence to be able to control the game. I think the reason why it's the most popular sport in the world is because it's so easy to play, most people can play it in one form or another.