Why is AMR not widespread?
Natural selection is environment-dependent. A mutation that makes an individual more fit in one context, might make it less fit in a different context.

The mutations that make bacteria more fit in an environment where they're exposed to antibiotics generally make them less fit in a "natural" environment where they don't have to deal with antibiotics. That means that MDR bacteria are often out-competed by their less-resistant competitors, unless they're in the "right" environment for them, such as a hospital or an infected, antibiotic-treated patient. 

Some references:


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Gagneux S, Long CD, Small PM, Van T, Schoolnik GK, Bohannan BJM. The competitive cost of antibiotic resistance in Mycobacterium tuberculosis. Science 2006; 312:1944–6.
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