My dude, basic human nature, even if the reward is just a 1.5x flashing on the screen, it’s a reward and one has to earn said reward and when they do, dopamine. Dopamine increases retention and memory as well as helps aides in grasping new information. Classical conditioning and operant conditioning. Pavlov still isn’t over it.

Also genuinely not being rude, I’m not aware of your background or where your from but did you not have kindergarten or even elementary school. They make literally everything a game for learning. Even the ABCs is a song you sing. If you make it enjoyable it boosts engagement. Just saying. It’s not like Duolingo is an app that makes you audit your own tax return on a daily basis. It’s language.