Coal miners are in great danger as well, both from accidents and long term exposure. It is expensive to move from source to destination. It is cheap. It is abundant. The second most abundant liquid on Earth.
Natural gas is higher temperature.
Unlike coal, which is terrestrial , coming from swamps, oil is from continental shelves and restricted basins in the Ocean. They require that they form in a permeable rock. They float up and must find a trap cap rock. Mesozoic and Cenozoic in Age. Older oil is probably limited because it has already escaped.