Why do antibiotics kill bacteria and not human cells?

Because bacterial cells and human cells aren't the same. Penicillin works by interfering with how bacterial cell walls are built, and human cells don't have bacterial cell walls. (Turns out all bacterial cells aren't the same, either, but penicillin works against a lot of them.) One of the challenges in medicine is finding antibiotics that work against bacteria's Biology, but that don't interfere with human biology. This is called selectivity. It's a really important principle of medications against all infections (you want the drug to selectively kill the infecting organism instead of your own cells) and against cancer, too (you want the drug to selectively kill the cancer cells instead of the healthy ones).