You have to remember that HIV-1 and MSRA are very very very very different creatures
HIV-1 operates as a 'quasispecies'-- Like a cloud of viruses that are all the same, but different
When an HIV-1 positive patient takes an antiretroviral, there are naturally viruses, already in that patient, that are resistant to the drug. That resistant virus is usually 'less fit' when compared to the rest of the viruses, but in the presence of the drug, they are the only ones left 'alive'. So, even though the virus is 'less fit', it can still infect/replicate, and slowly compensatory mutations will make that drug resistant virus just as 'fit' as a 'wild' virus, even when the drugs are taken away.
HIV-1 'hides' in cells. If a non-resistant virus is 'hiding' in a macrophage as a provirus (hiding in the macrophages DNA) while the patient is on drugs, its safe. These latently infected cells can spontaneously start producing viruses.
So, say you have an HIV-1 patient. They initially responded to anti-retrovirals. Their viral loads started going up again. They responded to second generation drugs. Their viral loads started going back up. Your treatment options are disappearing... so, you take them off drugs completely. Those 'ancestral' viruses that were hiding in cells during treatment are 'more fit', so they can beat down the antiretroviral resistant viruses. Viral load goes up, but its treatable again. This treatment scheme is related to evolution as a theory, the 'competitive exclusion principle'. Given a set quantity of essential resources (ie CD4+ T-cells), only the 'more fit' variant will survive/reproduce/etc. If you take away all the antiretrovirals (before the resistant guys can compensate) the non-resistant guys will out compete the resistant guys. Then you can use antiretrovirals again (kind of, not as well, but better than nothing).
But if you have a patient that gets to this point, theyre in trouble.
MSRA doesnt behave like this. Bacteria dont 'hide' during antibiotic treatment. The resistant guys are only 'less fit' in the absence of antibiotics... but there is *never* an absence of antibiotics... unless we start using phage therapy.
With bacteria, 'bigger' is better, because you want to wipe out EVERYTHING. HIV-1 is trickier.