Why are antibiotics not effective on viral infections?

Only probably because they are using the wrong antibiotic and/or the viral infection somehow evolved and protects itself from the antibodies created by the body because of the antibiotic.

According to betterhealth "Antibiotics are useless against viral infections. This is because viruses are so simple that they use their host cells to perform their activities for them. So antiviral drugs work differently to antibiotics, by interfering with the viral enzymes instead."