Because it doesn't sell. That's the problem.
They don't make a cohesive game with a vision behind it, so it bombs. Hence the examples of FF13, RE6, and even recently the likes of Mafia 3, or the first Watch_Dogs, which tickboxed things that they thought should be in a game (e.g. "what would Grand Theft Auto do?) like that rather than just go with their own vision.
The big, great titles that come out all have that vision. From this year alone, look at Doom - a remake but avoided all the modern grey military shooter tropes. Witcher 3 - from the very start of that series, absolutely redefining the western RPG genre. Overwatch - taking the multiplayer MMO shooter genre and avoiding having a load of army guys in it (indeed, one character takes the absolute piss out of that trope), making a bright, brand new, character driven shooter that has been massive. Dark Souls 3 - completely refusing to dumb down the difficulty levels and instead fully embracing, from the off, creating a new genre of its' own that didn't exist beforehand.
And what do all those titles have in common? Massive sales, awards all over the shop, while the hundreds of Call of Duty clones (including CoD itself) are ignored, and paint by numbers GTA clones like Mafia 3 are laughed at, because they have no underlying vision or passion behind them.
The market responds to what is good. Make a good game, they will buy. Niche titles like Stardew Valley - made by one guy where you build a farm basically - sold over one million copies in two months because it was simply exceptionally made and had a passion behind it. Not one bit of focus tested nonsense. Then look at Duke Nukem Forever - a game that tick boxed everything they thought they had to have in the game and then spent absolutely no time whatsoever on making it actually good, because they had no vision for it whatsoever.
How many zombie games are on Steam? Or survival games? Or first person military shooters? Literally thousands - and 99% of them absolutely terrible, because the developer had no vision beyond "zombie games sell so I'll make one of them." It's a stupid mindset to have.