Question

Why is the top most layer of the atmosphere thin?​

Answer 1

It is thin as we go up the layer of air column reduces...

Answer 2

Answer: The air in the exosphere is very very thin, making this layer even more space-like than the thermosphere. In fact, air in the exosphere is constantly  though very gradually leaking out of Earth's atmosphere into outer space. There is no clear-cut upper boundary where the exosphere finally fades away into space.

Hope it helps!!