Question

Why does the atmosphere weigh so much

Answer 1

its all because the gravity, is really heavy

Answer 2

Because the Earth's gravity acts on the atmosphere and holds it down, just as it does all other objects. The weight of the atmosphere is estimated to be about 5.75 quadrillion (5,750,000,000,000,000) tons. Air pressure is the "push" of the atmosphere on the Earth's surface.