Why should i take amoxicillin for uti for 7 days?

Depends...: Nowadays, the usually uncomplicated UTI only needs 3-5 days of taking antibiotics. For adult males, oftentimes, UTI would not suitable to be classified as uncomplicated UTI. So, ask the treating doctor for individual specific clinical situation, then decide how long you need to take the antibiotics.

To kill bacteria: Duration of antibiotics are set depending on the location of the infection and the severity of it. The patient should finish the whole course of antibiotics even if they felt better to avoid recurrence of the infection and to avoid resistant bacterias.