system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] Why do fossil fuels not replenish themselves?

So, according to wiki, the theory is that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the Earth's crust over millions of years. So are fossil fuels constantly being recreated by new plants dying? Was there just way more plants to create fuels long ago? Are we simply using them faster than they are being replenished?
gpt
In the Carboniferous geological age, a plant underwent a mutation allowing it to make a new type of structural support called lignin. (You might know it better as the main reason wood is solid - before it, plants were ferns made of cellulose.)  For sixty million years nothing on this planet was able to digest lignin, meaning that dead trees just piled up... everywhere.  Tens of meters thick around the entire globe.

This ungodly amount of tree locked up a huge amount of the atmosphere's CO2 and oxygen levels rose accordingly.  Wherever this carbon sink was buried by geological processes, it was subjected to heat and pressure, changing it into coal.

The Carboniferous ended when some fungi evolved a metabolic pathway to digest lignin. No longer would trees fall and remain, unrotting.  Coal is not replenished and will never be replenished because trees break down.