At the level the pros play it its a sport.
Its even more complicated then some sports, you have playing defense and offense just like either other sport, they both involve lots of strategy the same principals of sports apply in competitive gaming ( defense wins games). There are also other things that take effect like pushing power weapons and in games like halo controlling spawn points, timing weapons, team shooting, distractions.
Most players have their own role just like in real life sports, support player, beast player, team leader to call out strats and everything.
Competitive gaming can sometimes be more intense than real sports. theres no such thing as a time out unless you count the 5 seconds between rounds.
Players need amazing hand eye coordination as well as amazing teamwork and communication.
Competive gaming has some of the best communication which is one of the biggest parts of a sport.
Seriously find me a good basketball team that doesnt communicate when they are on the court and i will give you 50 bucks.
Its not like anyone can be amazing at video games, theres lots of people who naturally have more skill at it than others but even they need to train and learn new things.
Im not just a nerd who is in love with competitive gaming either(i do love it though) ive been playing sports my entire life, basketball, baseball, football, golf - competitive gaming is much more of a sport than golf although i do love golf.
If you can make a living off it, then its definetly not a hobby.
Competitive gaming has so much more depth than people see, i bet an actual pro could go into so much more detail than i did.