We are creating new versions of what we have already. When you hear something referred to, for example, as a "beta-lactam antibiotic", what that actually means is a more creative version of penicillin. We tack new functional groups onto antibiotics to try to outpace resistance mechanisms, but its a stalling tactic, not a solution.

New antibiotic classes are hard to find. You need to find compounds that are lethal to bacterial cells (but not our cells) that there's no pre-existing immunity to. That's an extremely tall order, and we've run out of easy sources for those compounds.