actually need to build immunity to it, we cannot vaccinate people every 6 mths.
You cannot know at this still early stage, if it's possible to 'build immunity' to it, and certainly not how long that might take (years? decades?).
Further down the line we'll have better knowledge and understanding, more of the rest of the world vaccinated (morally the right thing to do, but also reduced risk of dangerous variants). And wider availability of the new treatments. The antivirals and the monoclonal antibodies like sotrovimab.
Meanwhile, dead people can't build immunity, and others like the poor young man in the article above have been left long-term disabled.
Vaccines help build immunity. No, we can't keep boosting all the time, but we can (as the experts including the WHO say), eventually use a multi variant aimed vaccine (which is what they're working on).
In the meantime, because we're not there yet, we take simple and easy mitigations. Which obviously aren't 100% effective but work to mitigate eg. reduce unnecessary risks.
That is, if we want to keep the economy running successfully (and without mass body pile ups) and get back as much normality as is possible during the extraordinary circumstances of a pandemic.