Have you tried looking at stars on the horizon? It gets increasingly harder the closer you get the horizon. This is due to both light pollution and the previously mentioned reason.
I have indeed looked at stars on the horizon. In the absence of light pollution and on a clear night you can see stars perfectly on the horizon.
Here is a photo I took. On the sides away from the city lights you can see stars almost touching the horizon, look at the right side.
There is nothing at all stopping anyone from clearly seeing things at the very edge of the horizon, the stars are just as visible.
The stars, boats, buildings and mountains don't vanish because they get dim or are blocked by 'thick atmosphere', they vanish because they are below the curve of the Earth.
