Because unfortunately the current generation of vaccines doesn't completely stop the spread - they certainly slow the spread in the weeks after the second shot, but then antibodies start to wane over the next months (which is why booster shots are being administered). Vaccines still protect from hospitalisation and death, though.

If you believe most virologists the only realistic scenario is that the virus becomes endemic, similar to most flu strains or other human corona viruses (which makes vaccination even more important!).