Why are fossil fuels classified as nonrenewable energy sources?

Fossil fuels are classified as nonrenewable energy sources because they are formed from the remains of ancient plants and animals over millions of years. The process of formation takes an incredibly long time—far exceeding human timescales—meaning that once they are extracted and consumed, they are not replenished within a human lifetime. Hence, their depletion occurs much faster than any potential for their natural regeneration. This characteristic is what fundamentally distinguishes nonrenewable resources from renewable ones, which can be replenished more quickly, such as solar or wind energy derived from current environmental processes.