It's well known that bacteria live and reproduce outside the human body. Many strains, like listeria or salmonella, can live in food, outside of any human or animal host. Cholera lives in sewage. Also, animals are reservoirs for a large number of bacterial species. Unless we're going to dose all humans, all animals, and the environment as well, then we're not going to succeed.
Further, 80% or so of most antibiotics get excreted right out of the body in urine within a matter of hours. This means that the environment is going to get dosed with a low level of antibiotics, just the perfect environment for breeding immunity.
Finally, bacteria can exchange plasmids inter-species, so if some innocuous bacterium in the environment develops immunity, it can transfer that to a virulent species at a later time.