Evolution does not actually gift the Carkol with all that much additional coal, especially considering the increased energy needs associated with their new body size – but it does give them both the knowledge that their energy is finite, and, more importantly, a means of obtaining it from the outside world. Carkol think on the timescales of their rock-type brethren, and load their bodies up with piles of coal that are ordinary relative to their body plan, but would take thousands of years to burn. Before industry, perhaps it might take that long to escape an exhausted mine and find a new energy source; the coal dropped along the way warmed Galarians through the region's cold winters.
But for the last few centuries, even "wild" Carkol have developed a sort of symbiotic relationship with coal miners, who load their hauls onto its back and let these pokemon carry the coal up to the surface, burning only a tiny fraction of the coal in the process. This relationship met with bitter protests from the poor, who saw in the taming of the wild Carkol a scheme to jack up the price of home heating. Although certainly not the primary reason, for using Carkol was far more efficient than hiring miners or fighting pokemon to push a cart, it is doubtful any coal barons mourned the possibility.
Interestingly, in Galar's mining towns there are invariably legends of Carkol which did not evolve from Rolycoly at all, but instead were actual mining carts that eventually gained sentience. Skeptics tend to think of these tales as fictions to cover up the thefts of mining carts, but it is a rare mine that uses carts where live Carkol are available, and Carkol are far more expensive to obtain...