We do develop immunity to other coronaviruses such as the ones causing common cold - thing is, there are so many different viruses (some of them are coronavirus

We do develop immunity to other coronaviruses such as the ones causing common cold - thing is, there are so many different viruses (some of them are coronaviruses, some not - there's a huge variety) causing similar symptoms, so we don't even attempt to diagnose which of the many common-cold-group viruses you have because we don't care; but you do develop immunity against that particular single virus.