It is considered to be a sport by the majority of people, having all the attributes that any other sports has. There are defined rules, there is a level of competition, you have to increase your skill level to be able to improve and win more. There are fans like any other sport who watch and support their teams and players. Teams have uniforms and are at the top level paid a salary to compete as well as prize money for winning. There is very little reason not to consider it a sport.