because it's the psychology of spending. When you spend money, your chances of winning increase. When this happens, your brain releases dopamine and associates winning with spending money and you keep playing. The more you play, the more you begin to lose so in order to reactive those receptors, you spend money. Then you win a bit more but then you start losing. It's a vicious cycle to keep the average person at a lower elo so they spend money.
This is an actual algorithm for generating money. It's in every game. Magic the Gathering was the first game to do this with their packs to create an addictive gambling effect. World of Warcraft with the random item drops. Same thing. Now everyone does it. Hearthstone. Same problem. League of Legends same thing.
Try playing 25 games within two days and see what happens.
If you play too much for free then you get punished for it if you're not spending money.
When people lose, they gotta keep playing to make up for their losses but when they keep losing they want to feel better so they spend again.
It's a psychological trap. The shitty thing about online gaming.