Because the game doesn't have a "back to lobby" button to click in that situation, and the evaluation screen won't come up when that happens (which means, I can't rate you as a poor player, darn!), so it effectively locks up.
Likewise... When your hand consists of a single card, and you've played that, there's no reason for your main phase to drag on for another minute. Yes, I'm impatient. I like being efficient. If my turn drags on, it's either because I have a messenger window open (okay, bad me... I never said I couldn't be hypocritical) or because I'm thinking about what to do. With one face-down monster in defense position, and nothing else to your person, how much thinking can there be to do?