The vaccinated only grow further immunities if they become infected by COVID and the body has to respond. IF the vaccinated get the illness, while the antibodies are high, they get a very mild case and in many instances never have to recover. This is why they do not create long term immunities. Once those antibodies subside, their bodies act as if they have never seen the virus as they did not create long term immunities (B and T cells in bone marrow).
Its a bit more complex than that but that is the general description.