It's because of Underrealm Lich. Per the card's Rulings: Underrealm Lich’s first ability applies even if you have fewer than three cards in your library. If you have no cards in your library, you can’t lose the game for trying to draw a card from an empty library.
Wizards Megan
It's because of Underrealm Lich. Per the card's Rulings: Underrealm Lich’s first ability applies even if you have fewer than three cards in your library. If you have no cards in your library, you can’t lose the game for trying to draw a card from an empty library.