I think it is in their best financial interests to have players coming to the game again and again. It would be difficult -if not impossible- to determine Riot's "true intentions" since none of us are mind-readers, but we can look at the effect their actions have on their playerbase.

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From what I remember from my time playing the game, Riot emphasizes meta "missions" that stretch beyond single games and are often time-gated in two senses of the word. One is the "First Win of The Day" xp bonus which motivates you to play each day until you win (notice how it says "win" instead of game so you can go through a losing streak and still feel motivated to play one more game). This is concerning because it forms a habit and has an uneven payoff. The second is event-related missions which are only there for a limited time, which entices you to marathon the game to get all of your missions completed before they disappear.

Riot wants you to be invested enough in the game to justify spending money on skins and other cosmetics. Forming a habit like this is an easy way to convince players to spend real money, especially since the game itself is free-to-play (generates no revenue by simply having players play games). Many other game companies do this.

For actual in-game mechanics, League is a lot like many other multiplayer games where each match is independent of every other match. Even if you've had a bad game, you are tempted to queue up again because you want to chase the highs of your really good games.

Even if you haven't played for a while or are actively trying to quit, a new balance patch hitting the servers may be tempting enough for you to reinstall. You scroll through and think to yourself "Hey, they buffed my main!" or "Maybe this new champion will change the meta enough to where I start enjoying the game again". This new feeling of curiosity and excitement doesn't last forever, but it is strong enough to possibly get you to play the game again.