Why do scientists believe that fossil fuels will run out?
Because they are an exhaustible resource.
Fossil fuels are not reusable. Unlike water (which has a cycle), fossil fuels get used once, get turned into energy and byproducts, and cannot be returned to their original state. And unlike wind and solar power, fossil fuels are not limitless. We only have what is physically in the earth, and the earth can't make them as fast as we're using them.