Immunity came about because if prosecutors were Personably liable for charges where there were no convictions no one would take those positions. Similarly if police were personally liable for arrests where there were no convictions no one would be a cop.
But the same is true to a degree for all government, for example if I were to get hit by a county/city bus, generally the driver won’t be liable and worse my damages against the government are capped (I believe at $200k In FL, unless the government waived the cap and good luck with that).