Because it might be easily overseen by the aircraft(s) on its way to USA and thus never reach the destination :) Because it's difficult to locate it, if you do

Because it might be easily overseen by the aircraft(s) on its way to USA and thus never reach the destination :) Because it's difficult to locate it, if you don't use electronics to check on it but rather by the plain old good observation. There are a few reasons why it shouldn't be a color that blends into sky. If it's not hidden, then there's no need to hide.