Don't assume everyone on here is an "armchair biologist".
There are several problems with IV antibiotics, some of which have been outlined in other comments:
1. It involves an IV. This means a hospital - that's both expensive for all involved, and the presence of an IV is in and of itself a risk for bacterial infection. Not all antibiotics kill all bacteria.
2. "The vast majority" = / = the ones experiencing resistance that are of clinical concern. C. difficile lives in your gut. MRSA on the other hand can live on your skin, and colonize your nasal passages. Both, additionally, cheerfully live on surfaces for quite some time (really quite some time for C. difficile).
3. IV antibiotics can reach your gut - some are excreted into the intestines. Metronidazole is, vancomycin isn't for example. And beyond that, much of the concern is with active infections, which are very likely not in your gut for many diseases, in contrast to just the background of your intestinal flora.
4. You're asserting that antibiotics reaching your gut from an IV would arrive in low doses. That's a good thing for promoting the development of resistance, not a bad thing, from the bacteria's perspective.
There are several problems with IV antibiotics, some of which have been outlined in other comments:
1. It involves an IV. This means a hospital - that's both expensive for all involved, and the presence of an IV is in and of itself a risk for bacterial infection. Not all antibiotics kill all bacteria.
2. "The vast majority" = / = the ones experiencing resistance that are of clinical concern. C. difficile lives in your gut. MRSA on the other hand can live on your skin, and colonize your nasal passages. Both, additionally, cheerfully live on surfaces for quite some time (really quite some time for C. difficile).
3. IV antibiotics can reach your gut - some are excreted into the intestines. Metronidazole is, vancomycin isn't for example. And beyond that, much of the concern is with active infections, which are very likely not in your gut for many diseases, in contrast to just the background of your intestinal flora.
4. You're asserting that antibiotics reaching your gut from an IV would arrive in low doses. That's a good thing for promoting the development of resistance, not a bad thing, from the bacteria's perspective.