You are taking for granted that understanding how to follow game mechanics and taking turns are both acquired skills. It's very rewarding for children to learn those skills and see themselves apply them. The fact that there aren't interesting decisions to make in playing the game is secondary.

The kids are spending all of their mental energy making decisions to follow the game mechanics themselves instead of moving out of turn, jumping to the squares they want to land on, taking extra cards, etc. All that requires a lot more executive function and willpower than we realize because we're so practiced at it, but Candyland is a way for small kids to get that practice.