Dominion is actually one of the simpler deck-builders out there. Games like Ascension or Star Realms have more going on with their market rows. Your niece probably picked it up because the core loop is just buying cards and playing them. It's easy to overthink the strategy, but the rules themselves are straightforward. Once you get past the first few turns, it clicks.
What if the real problem is how we teach games to adults? We come in with all this past experience and try to map every rule to something we already know. A kid just sees the pieces and follows the steps. They don't worry about making the perfect move right away. Maybe we make games hard by trying to win on the first turn instead of just playing.