Most existing coal was once trees, but large scale conversion of trees into coal was only possible in the carboniferous, when trees were new on the biological scene and there wasn't anything that could decompose them, so dead trees just piled up and slowly turned into coal under the anaerobic conditions of being buried under other trees.

You can still get coal made from trees, when a forest fire turns some wood into charcoal and that gets buried in the soil, or under unusual circumstances where a tree gets buried somewhere inhospitable to the microbes that can decompose lignin.