A couple issues: 1. It's hard. You need to find a molecule that has a mechanism of action against bacteria, but only bacteria. Any cellular function you share

A couple issues: 1. It's hard. You need to find a molecule that has a mechanism of action against bacteria, but only bacteria. Any cellular function you share with human cells is right out (this is why anti-fungals are double-plus hard mode). We got away with this by glueing new functional groups onto things to evade resistance, but inventing a new target never before seen in nature is...tricky. Most of the easy stuff got developed by the bacteria first - they've been working on this problem longer than we have. 2. It's very expensive. Fidaxomicin (aka Difficid) cost $175 million to develop, and it's on the cheaper side. While it might still be profitable, that's a steep up-front cost. One does not casually embark on developing a new antibiotic.