I assume that they would have used up the easily accessible fossil fuels - that stuff was ancient even to the dinosaurs. They could have left something on the moon if they wanted a more lasting monument to their existence though.
The easily accessible fossil fuels got to the surface somehow for us. Maybe given enough time and geologic changes more gets exposed. So the coal we started with used to be hidden, and wasn't touched by an earlier civilization.
I don't think they're all that exposed to us as is, and we're constantly wringing out more and more as technology improves, but you have a good point - it's a big planet still and geologically active.