I remember this forum called TCGApp (it's probably still around) I used to visit to check news about Ayakashi Ghost Guild, a shitty collectible non-trading card game made by Zynga.
The game is basically Button Clicker Simulator 2012. You recruit cards by paying in-game coins, extremely expensive real money coins or by winning events (which used to be possible before Zynga went full Jew with it and made impossible winning unless you paid money to click a button more times).
The game was boring as fuck. All you did was clicking a button until you found something, then "battled" with an enemy by clicking another button a single time. That's it, there was no strategy whatsoever outside of team building.
Programming-wise, it was a fucking webapp with horrible performance that crashed constantly. It was as shitty as you could imagine.
Thing is, the artworks were pretty cute. It was the only redeeming quality of the game, which is why it managed to become successful, I guess.
I remember reading that forum during Christmas' events. There were around four users claiming they spent around $2500 that month, but apparently they spent $1000 monthly on that game.
These same people complained about the Asian cash cows. It was a known fact that they got far more offers than the rest of the world necause they spent much more money on the game.
So there is how a game makes money. Put in some feature that can attract autists with too much money on their hands, like cute 2D girls, and you will be set for life.