In fact, the mechanism for breaking down both cellulose and lignin are quite complex, which is unsurprising given the complexity of these molecules. Cellulose is at least a relatively simple polysaccharide that will break down by hydrolysis in a mild in polar solution, which you can observe by putting a piece of uncoated paper in water for a few hours where it will basically revert to pulp. Lignin, on the other hand, is a complex polyphenolic macromolecule with complex cross-linkages. It is not soluble in water or simple alcohols, hence why hardwoods are such a good material for storing or protecting from water, especially when treated with creosote or another agent that inhibits fungal growth. The degradation of lignin structures requires not only a complex enzyme (typically lignin perioxidase) but also other metabolites and an energetic fuel (in this case, hydrogen perioxide) which also has to be produced by the organism breaking down lignin. We don’t really even have a comprehensive understanding of lignin biodegradation at a detail level. As far as I’m aware, plants which use lignin do not produce enzymes to break it down, hence why trees continue growing upward and outward indefinitely (unlike animals which are in a constant state of breaking down and replacing many tissues on an ongoing basis). There is a lot of energy stored in the macropolymers that make up lignin, but the material is very robust. Once might as well ask why organisms evolved to directly consume coal or petroleum haven’t spread across the planet.
Antimicrobial resistance, on the other hand, is a relatively straightforward manner of changing the epitrope by random mutation which prevents an antibody or lymphocyte from binding to it. Because this is often encoded in plasmids or fungible codon sequences it can be easily shared by lateral gene transfer, hence why populations of antibiotic resistant bacteria can spread so prolifically. It only takes a small colony of successful (e.g. resistant) microbes to populate a host organism and then spread to others by any number of vectors, and that spread, as with viral infections, will be geometric progression until it hits some limiting factor (no more hosts, containment of the population, natural resistance, et cetera.) Bacteria become resistant to antibiotics, or more specifically, to the response of the immune system, because that’s what they do; they grow and mutate as fast as resources will permit, and occasionally those mutations will impart resistance.