The atmosphere is more dense at the surface and thins out as we climb altitude. Several gases are probably denser than air so there’s few of those as well at altitude. Heat is concentrated where the air is more dense and also heat trapping gases like water vapour and CO2 are abundant at sea level.
But I am just testing my brain and could have done a search to back up my answer. That means I could be wrong.
Reads like some CEO is impatient