You know, I used to think that (and I still do, to a large extent), but:
1. They do put a lot of time & effort into playing games. I mean sure, every kid out there spends hours playing video games, but these guys really take it to the next level, like how concert pianists train. That requires at least some degree of dedication outside of the normal braindead mode I go into on weekends playing games on Steam for hours on end, lol.
2. I cannot compete against a 14-year-old at online gaming anymore. My reflexes simply aren't fast enough. Playing Counter-strike back in high school online was a vastly different experience than playing the same game now because I've slowed down physically & mentally over time. Not that I was some kind of elite-level gamer or anything, but mental & physical reactions definitely play a large role in e-sports.
2a. I've read some studies where they've found that elite gamers can physically track more points on the screen than regular people can, among other advantages. So there is apparently a level of separation in reflexes, mental acuity, etc., which technically qualifies as a sport. I mean, some countries recognize chess a sport (where you're using your mind instead of your body) & some don't, so the definition itself is still up for discussion. And it's apparently under consideration for the Olympics:
https://en.wikipedia.org/wiki/ESports#Olympic_Games_recognition
3. You can make money at it, you can make a living at it, you can get sponsored, you can even make advertising money off of it. PewDiePie has 63 million Youtube followers & a net worth of $20 million just from playing video games online. League of Legends in Korea apparently offers salaried positions for professional players. It's a mutli-billion-dollar industry. Wikipedia says that gambling on Counter-Strike skins alone was worth an estimated $2.3 billion in 2015