Bacteriophages need direct contact with bacteria (somatic and pilus or else). Oposite to chemicals (antibiotics or disinfectants) that are soluble and can reach bacteria from all directions ! Another point is the genetic drift that creates at certain probabilities mutants that are resistant to phage infections. Phage therapy is excellent as long phages are able to infect specifically (contrary to antibiotics, their concentration increase with host numbers while antibiotics decrease no matter how many bacteria are present...)

Thanks... But i still do not understand why they don't clear infections naturally if they are already on site??