Earlier quoted context omitted.
Is there reason to think that bacteria with every gene for antibiotic resistance would be at all viable? It seems likely that these genes have other negative effects on fitness that make them unlikely to spread significantly in the absence of the antibiotic.
The tendency, under selective pressures, is for organisms to become more fit over time. By applying constant antibiotic-induced generalised selective pressure, you're greatly increasing those odds
The fact that the gene is not already widespread suggests that in the absence of the selective pressure of the antibiotic it has other effects that decrease fitness in a typical environment.