This is a really good point. A lot of the biggest games -- especially big multiplayer games with some kind of open-ended systems -- aren't really "games" that are part of a "gaming" hobby; they're a hobby unto themselves. You see this a lot with games like League of Legends or World of Warcraft -- or outside of digital in games like MTG or poker: if a game is rich enough, competitive enough, and fun over long enough periods of time, a large portion of its fans will be people who just use that one game to fill all their hobby time and don't even consider other similar things.
You also have to come in with a really impressive offering that is significantly differentiated in order to even attempt to compete, since they don't have a clear end point a la a singleplayer game, and thus their fans will only try things that look really exciting to them.
Like, we can go through a few of your examples and look at the couple of instances where there were actually successful competitors.
MOBAs: Dota 2 had a very consumer friendly business model, and was a sequel to the predecessor to League of Legends (which pulled people off by having a vastly, vastly superior way to play a MOBA than the insanely clunky Warcraft 3 mod). Blizzard had a game that played very differently, had a huge install base of fans already, and was made by the team that made the games the genre itself is derived from. There were some minor successes in games like Smite and Awesomenauts that targeted a niche audience that wanted a large departures as well (2D sidescroller MOBA, third person action MOBA), but their audiences are certainly still tiny compared to LoL. Straight clones like Heroes of Newerth or the legion of games with subpar quality simply withered and died.
MMOs: We primarily saw success with either very unique titles with more niche audiences (EVE), games that completely upended the business model and gave out their game for free, or games with ~$200+ million budgets and huge brands (SWTOR, TESO, and FFXV all had modest success in comparison, but were able to sustain sizable teams while generating healthy profits, and two of them still had to change their business model in the end).
MTG: Hearthstone managed to deliver a much faster and cheaper game to play that worked great in a digital format and used the strength of Blizzard's brand and audience. It was also easier for new players to understand in quite a few ways, streamed well, and was widely recognized as being of very high quality.
Poker: I'm not sure what the comparison point would be here. Slot machines? Horse racing? Other forms of gambling? I guess there are a few variants of Poker itself, but basically they had to match the sense of suspense and either offer a completely non-social/non-thinking option (slots) or an even more social environment (sitting at a race track, thus being able to talk more openly than focusing on your cards).
For singleplayer, we do sometimes see similar phenomenon, and the example I like to use is GTA. When people first started competing with GTA, they just made GTA with a new skin, and those games had pretty mixed success (and are basically all gone now). Later on we got games like Assassin's Creed instead, which have huge structural similarities in how they work, but feel very different for the end user. It avoids the me-too sensation while targeting the same audience.
Or, to put this in shorter form, if you're not financed and stacked with talent to the gils, you shouldn't be trying to take on a competitor directly, and instead trying to build a decidedly different product that appeals to a similar audience. You should also pay very careful attention to what you could feasibly do. For the vast majority of developers, you want to be making EVE or Awesomenauts instead of throwing half your company's money at the wall, or simply look at an entirely different venue all together. Of course, there's only so many genres or platforms to work on, so at some point your game is going to be similar to or competing with something.
