An intuitive analogy is your gut is like a rainforest. It's hard for any one species to become overly dominant since it's a very dynamic system with many species that have optimized to even specific tree species. However in extreme environments like the Arctic there are only a handful of species as the environment is unfriendly to all. Antibiotic is akin to burning down a rainforest which does get rid of a few bad species, but also creates all the other species that keep the bad species in check. Post fire if a bad species is the first to take hold the entire area is fucked.

Your mouth has it's own floura. Sure you can blast everything away but eventually you'll be far more securable to some alcohol (or whatever antiseptic) resistant bad bug and you will get very sick for a very long time.