I think I’ve not worded my question well.
To rephrase: why are captive angiosperms often much more skilled at defending themselves from competing taxa than captive fungi and myxomycetes?
Sure, you have to remove weeds that pop up in your houseplant’s pot, but houseplants are normally able to resist attack from numerous soil microbes in the dirt quite easily.
Also, have there been any attempts to create laboratory conditions that encourage antibiotic-nonproducing mushrooms’ resistance to competitors when unsterilized? I know certain fast-moving myxomycetes can reduce microbial load by migrating (this causes competitors to be sloughed off and physically ejected from the slime, not sure how slow myxos that don’t make antibiotics do it though), and that many lichens are sun-resistant and can cook unspecialized pathogens to death by being dry and hot, but cannot find any references mentioning the anticompetitive mechanisms of most saprotrophic fungi that don’t produce antibiotics.