You couldn't be more wrong.
They are games with an insane amount of depth and you need to be extremely skilled if you want to play it on a level the likes of what you see in The International. Don't assume, this is something you'll just have to take for granted. Now I hear you think "but every competitive eSport requires the players to be extremely skilled!", and you couldn't be more right, but you would forget some key differences that make MOBAs much closer to a 'real' sport (and thus more popular) than any other eSport out there.
First and foremost; it's the only eSport where teams truly matter. Unlike first person shooters and fighters individual skill only gets you so far. I could go into great detail explaining why this matters, but if you know how teams in team-based sports like Soccer and Basketball function and are aware of the fact they are WAY more popular with the mainstream than most individual sports out there you should be able to understand why this is essential to explain the high amount of viewers and attention LoL and DOTA get.
Then there is its presentation. MOBAs are infinitly more watchable and, once again, more like some of the world's favorite specator sports than any other eSport out there. Matches can last up to an hour instead of mere minutes, there is a top-down view that makes it easy to keep track of everything happening (this is imo the main reason FPS doesn't really take of as an eSport even though every bro on the planet plays Call of Duty), there are great announcers who don't just know the game well but are actually great at their job announcing stuff and add a lot of fun and excitement to the game (don't understate the importance of this), the list goes on and on. Valve and Riot simply go above and beyond all other eSports by truly creating professional entertainment instead of just striving to be that. There is more to the modern MOBA than just appreciating the skill put on display, while the focus mostly is on the latter in other eSports. Just watch The International or the LCS for a bit and compare it to any other eSport stream out there to see what I mean.
Of course, like any sport, you need to understand the rules to truly enjoy watching a match of it. But while you can enjoy basketball if all you know is that the ball needs to go in the hoop, you can't enjoy chess if you don't know every single rule there is to it. Most eSports are the former, MOBAs generally are the latter. This is why a lot of people like the OP don't get MOBAs: you can't truly enjoy them if you only know the basics. This is true for both players and spectators. You even need to be at least vaguely aware of the powers every champ in the match brings to the table or you'll be completely puzzled once any actual fighting happens. The more you know about a MOBA, the more fun it gets to watch/play, which is why spectators/gamers who stick with them are so obsessed by them. Hell, I've played LoL for more than a thousand hours and even I am still learning stuff.
There is also a lot of diversity. MOBAs contantly change, grow and evolve without having to change its core mechanics (fighters) and there are so many variables in-game that not a single match is the same. That last thing is once again something that makes it more like a non-digital sport. Real life pretty much has infinite possibilites, games don't because that would be a bitch to code. Most eSports come down to who is the best at a few techniques generally accepted as the best while there is still lots of creativity in MOBAs. The former is the reason StarCraft isn't as popular as it used to be. The limit of what is possible is the absolute highest of any multiplayer game out there at the moment, period (except for maybe Minecraft, but guess what game is the subject of the most popular channels and videos on Youtube... ;D).
I could go on for a while, but other big factors are its community (the non-toxic one that is, check out the LoL subreddit for a good example of some of the people you can meet besides angry 13-year olds), friends playing/watching them together (and encouraging anyone they know who doesn't to do so), and boobs (only half joking, just look at every female champ in LoL and the crazy amount of female LoL cosplay. People (read: young males) love those things. Also brings it closer to real sports by the way, just go to pretty much any page in the FIFA World Cup threads for proof in .gif).
tl;dr: Other games try, but MOBAs are the only eSport that come close to having the same formula behind the succes of the worlds biggest spectator sports (Soccer, Basketball, you name it)
