Explain, why are fossil fuels classified a non-renewable sources of energy?

Fossil fuels are obtained from the buried plants and animals under earth over millions of years. This phenomena happened due to slow changes in earth processes. Thus chances of reoccurring of such processes is very rare and after the present stock is exhausted, we cannot hope to get new stocks unless there are drastic changes in the earth processes. Hence, fossil fuels are classified as non-renewable sources of energy.