Why are many fossils found in deserts?
Bones don't last very long in jungles. Or in forests. Or almost anywhere.

Fossils are the consequence of one highly unlikely fluke after another. Darwin himself commented on this. It takes just-right circumstances to have the bones of a recently deceased animal not be eaten by scavengers or turned into rot by bacteria. It takes yet another set of just-right circumstances for those bones to turn into fossils. It takes yet another set of just-right circumstances for those fossils to be brought back to surface level. It takes yet one more set of just-right circumstances for those nearly exposed fossils to survive the ravages of time and weathering.

The last coincidence is where deserts come into play. Water is an extremely powerful weathering agent. The key characteristic of deserts is an extreme lack of water. Fossils are much more likely to survive the savages of time without that weathering by water.