I wouldn't know why those who already have immunity would get the vaccine. There are studies showing that a shot post Covid-19 greatly increases the immune response agains variants, so there could be reasons.

In Europe getting the decease and beating it is considered good enough for immunity. Talk to your local authorities if you are not happy with your local practices.

But only last for 6 months, hence the boosters. Natural immunity lasts longer.

You can always chase people who sneeze on the streets(hang around a hospital maybe?), get your covid, test positive, quarantine yourself for 2 weeks and hope for the best as an immuniser or a natural booster.

The vaccines on the other hand are much more practical and safer. They seem to lose some efficacy over time against getting covid but the results against getting seriously ill are very promising. Which probably means, if you get Covid-19 naturally later on, it would be a natural booster with much lower risk compared to a non-vaxxed person.

All immunity is natural; vaccines don’t install a new immune system.