If someone has strep several times and they now have a case where amox, penicillin, zpack and clindamycin aren't working should they just let their body fight it off naturally at that point?

If proper culture and sensitivity test are performed the appropriate antibiotic should be able to be chosen

In such cases diagnosis has to be questioned and treatment should be guided on cultures results and/or non infectious causes if applicable. Assuming that the cause is always infectious and caused by same bacteria would be risky and exposing you to complications of possibly missed or untreated conditions.