I can’t answer the “why do captive fungi and myxomycetes seems less capable of self-defense?” question, but I can speak about plants.
It may have to do with scale at older ages. A young plant honestly does quite poorly at defending itself from the world/competing organisms. In the horticultural trade, plants are often started in sterilized or sanitized* soil, and kept under very careful conditions. There is also a lot of fungicide, insecticide, and herbicide used (unfortunately and often unnecessarily, in my opinion). When you are buying your houseplant from a nursery/other retailer it is at a larger and later life stage and so has more internal resources to fight off a small amount of fungi or bacteria and might blow in on the wind, or in other ways, but its origins may have been as carefully crafted as a captive fungi’s little dish.
Additionally, such “adult” plants will still need to be kept in the right conditions: light, day/night cycle, temperature, humid vs dry air, soil moisture, nutrient levels, and so on for them to do well. Powdery mildew and rust infections happen frequently in my area. So does scale, aphids, and heck, even gophers, though we’re not talking about houseplants at that point. Depending on the plant species, powdery mildew can occur from as simple a reason as the leaves got wet, and the plant does not survive the resulting infection. Plants are genuinely pretty touchy and houseplants are specifically selected species known to be able to be grown in the conditions found in most homes. There are many plants that just won’t put up with those conditions, and need to be treated as carefully as if they were in a Petri dish.
Finally, sometimes plants are grown in sanitized soil* with additional specific organisms added into the soil as a beneficial partner to that plant. So while the plant is non-sterile, in a way people are putting in a lot of effort to ensure it has the absolute safest conditions for optimal health. More effort than just providing a plain sterile substrate, perhaps. So… I think plants aren’t as hardy as they first appear. Growers do a lot of work!
I hope my experience working in a native plant nursery was externally helpful for once! :)
*sanitized soil - what is that?
Where I worked soil was heated to about 140º, then held at that temperature for at least an hour. This tends to kill off the vast majority of detrimental microorganisms while leaving beneficial ones intact. I have no idea why beneficials seem to have this temperature cutoff, but it is a thing for sure! Here in California we specifically do it to prevent the spread of Phytophthora ramorum, the organism responsible for “SOD” or Sudden Oak Death, but it takes care of other disease causing organisms as well and allows us to recycle soil we already have on site. It’s wise to check if the nursery you’re buying from does this no matter where you live to help prevent “invasive microorganisms” and just to have healthy plants in general.