Fossil fuels are considered non-renewable because they take a long time to form. Specifically, fossil fuels like coal, oil, and natural gas are created from the remains of ancient plants and animals that have been subjected to heat and pressure over millions of years. This process takes an extensive amount of time, and the rate at which we consume these fuels far exceeds the rate at which they can naturally regenerate, making them non-renewable resources.
While it is true that fossil fuels are often found underground and that they are formed through heat and pressure, the key factor defining them as non-renewable is the lengthy formation process.