Question

why does the thickness of atmosphere does not remain the same

Answer 1

The Earth's mass exerts a gravitational force that holds to the Earth a large amount of gases, known as the atmosphere. The concentration of gases thins out as one moves away from the earth.
The thickness of the troposphere (lowermost layer of atmosphere and closest to the earth) varies around the planet. Near the equator, the troposphere is thicker than at the poles, since the spinning of the Earth tends to shift air towards the equator. The thickness of the troposphere also varies with season. The troposphere is thicker in the summer and thinner in the winter all around the planet. At the poles in winter, the atmosphere is uniformly very cold and the troposphere cannot be distinguished from other layers.