The mechanics were well disguised and hidden behind several levels of indirection. There's a game component which gives you resources and generally improves your standing in the game's community. You can't just play the actual game though; you need "stamina" to play, which is just a fancy way to describe a timer. If you run out of stamina, you can't play again until it regenerates. One of the reasons this timer exists is to rate limit the game. Your progression in the game can only happen at the fixed rate dictated by the timer. The timer resets every 8 hours or so and this acts as negative reinforcement, creating a log-in habit: people feel like they're losing progress and falling behind if they don't log in 3 times a day. The timer should always be running to ensure maximum gains; every second it sits at 0 is a second wasted. Lots of people wake up at 2 AM just to do some game tasks that became available because the timer reset. It's insane.
What makes it a spending competition is the fact you can pay to reset the timer. This means non-paying customers are stuck with a fixed progress rate while the whales pay to amass ridiculous amounts of resources quickly. The casual players who don't pay money only have a chance in the beginning. They simply can't compete over the long term so they quit the game. This leads to a constantly decreasing player base.
The game operators usually create hundreds of different servers. They stabilize over the long term, with small groups of powerful players dominating it. Eventually, they merge stable servers in order to make players compete with each other for dominance once more. The more merges a server's been subjected to, the harder it is to compete with the other top players and the more money you must spend just to keep up. You might have been a top player in the original server, but you fall by hundreds of positions after a merge, and you're pretty much forgotten after two merges. Unless you start spending increasingly ridiculous amounts of money, of course.
If they just merged old servers, eventually there'd be only one remaining. That's why they create at least two new servers at the same time. It forms a beautiful cycle. It would be perfect if every server started off with the same amount of players every time, but games lose popularity over time. Eventually, it will die.
I once thought the traditional PC/console video games industry would never design money traps like these. Unfortunately, this kind of thing is slowly becoming the norm in online games. They're not as extreme as the mobile game I described above, but these ideas do seem to be slowly creeping into modern online games. If this is what the future of video games looks like, I won't shed any tears when governments regulate it.