In a restaurant you can leave at any point with very small excuses; you can safely blame the cuisine if you don’t like it; and it’s a large environment with distractions, where one can simply “blend out” if he wants to. At somebody’s home the attention on each other is more intense, it’s a small and quieter environment, you can’t tell the host that his lasagna is terrible or his background music is atrocious, and it feels a bit offensive to leave early.
This sort of thing is not an issue if people are close friends who can safely slag each others’ off, but in an increasingly globalised and atomised society the ratio of friends to acquaintances has decreased significantly, so gatherings probably tend to feel more formal.