It's insidiously addictive, and not because it's fun. After a while you realize that strategy is pointless, and the game is designed to keep you wanting to play while making it extraordinarily difficult to actually get better. Hearthstone uses the same sort of psychological tricks and tactics found in Las Vegas casinos meant to keep people gambling and eventually lose all of their money. This isn't just some rando from the internet's conspiracy theory either, a few google searches will confirm.