Why do you see night and day?

Answer 1

Because you have eyes.

That, was mean. Ha ha, it's 'cause you have eyes. Wow, that was mean. Here is a more kind answer; Just because. The Earth, Sun and Moon are still moving, so we still have it. You see it because we move. Plain and simple.

Answer 2

The Earth rotates on its axis, causing different parts of the planet to face the Sun at different times. When your part of the Earth is facing the Sun, it's daytime. As the Earth rotates and your location faces away from the Sun, it becomes night.