Can any doc explain why can't our immune systems cure stds/stis?

Here are some: The capability of personal immune system is constantly changing and adjusting to reflect the sustaining level of personal mental & physical health, which chiefly relies on healthy lifestyle without overindulgence, so to kill/suppress any invading organisms - bacteria or virus. Further, medical professionals have never cured anyone yet but helped people self-heal themselves with residual strength.

Actually, they do!: The immune system clears up most STDs. Without treatment, gonorrhea and chlamydia usually are self cured by the immune system after a few weeks or months, but sometimes only after serious complications have occurred. Syphilis usually persists, but not always. HPV and genital warts are cleared by the immune system as well. Herpes is partly controlled by immunity, but not cured.