It's super complicated - clinical trials of phage therapy have been relatively unsuccessful for one thing, so there's that. But additionally:

  - Bacteria adapt to phage infections, just as they do to antibiotics
  - Many phages are very host-specific, so a given cocktail might not even be able to target the pathogenic bacteria in question. There are phages with broader host spectra, but then they might be able to target the rest of your microbiome, which can lead to other health problems
  - Since phages have protein shells, human antibodies will target them, and may prevent them from reaching the infected tissue. You might also only be able to use a particular phage once for this reason.

I could go on, but suffice it to say, there are lots of unknowns and lots of complexity. Still, I do have hope for the technique in general, and our ability to understand phage-bacteria interactions is only getting better now that sequencing is super cheap. But I wouldn't bet my life on phage therapy working if I needed it today - which is why it's generally limited to compassionate use when antibiotics have failed.