Night does not mean dark. It means the horizon may not exist
A night departure over water, unlit terrain, or into haze removes the horizon without warning. The sky and the ground merge into a featureless black. The pilot who looks outside sees nothing useful but does not immediately register that the visual reference is gone. The vestibular system fills the gap with sensations that feel like valid attitude information but are not. The pilot who has not transitioned to instruments is now flying on false cues without knowing it.