The venom is secondary. The primary reason is that our sleeping state provides the conditions of a good sanctuary for them, so they often get close in order to rest or nest and everyone gets spooked once we wake up.

To snakes, we’re warm and provide shelter.

To spiders, our open mouths, ears or even nose are hidey-holes that provide near perfect conditions for them to rest in or ambush prey.

Which is why waking up and finding them around is very traumatizing and often startles them into retaliating for self-defense.