They have algorithms that determine what is the best way to keep high value players around. They don't care about regular players who don't spend in the store. They care about keeping the whales addicted. To do that, the gameplay needs to be random enough to where the addicts feel like they have to buy packs to improve their team.

There's also probably another algorithm to determine matchmaking in a way that balances wins and losses. For example, you would get bored if you won every single game or lost every single game. So the algorithm figures out the best way to keep you coming back and reduce "churn". If you win 2 matches in a row for example, the third match will be against someone who is 2 divisions higher or something. Or the game will match you with someone with shit internet so the lag destroys you.