simple newb question

just a simple question: I notice in some pictures of "base camps" at higher elevations always seem to have sleeping bags on top the tents!? why is this??
thanks

Most likely they are airing out the bedding; it can get pretty rank after a few weeks if not aired out regularly.

Not just rank but also damp. All lofting insulation loses some effectiveness when wet so it is important to air out and dry out you bag if you are using it over a long period of time or if it gets wet. You can also see people on the trail doing the same thing during a sunny break if it has been raining for while or it is a damp climate.

You perspire in your sleep and this moisture gets into the insulation. After a few days, it needs to be dried out in order to retain loft.

Have you checked to see if there are people in the sleeping bags?

Everyone has it right. The key is that at elevation, it is generally quite dry (there are exceptions, for example, when I go climbing in Colombia, even at 4500 meters it is humid and wet). During the day it can be blazing hot at altitude. The conditions are right to get all that ice and moisture sublimated and evaporated out of the sleeping bag.