It's not so easy to evolve new enzymes, but one of the main hypothesis is that at the beginning of the Carboniferous https://en.wikipedia.org/wiki/Carboniferous the plants invented lignin (that is the browns part of wood, that is mixed with cellulose that is the white part) and nobody knew how to eat eat, so for 50 millions years the dead trees accumulated on the floor and later transformed to the current mineral coal.

Also cellulose and starch are very similar, they are chains of glucose, but bounded slightly different. We can split only starch, but we don't have enzymes to split cellulose, were "we" includes cows and a lot of animals that would really love to digest cellulose.

I think some bacteria can digest weird sugars, even the mirrored versions, but they are more efficient digesting usual sugars.