Because that's not how it works. The vaccine makes antibodies and primes your T-cells to expect the viral antigen, both. Even if you had some weird response and failed to make antibodies, your T-cells would protect you. The T-cells are part of the immediate response to infection. It takes a while to make antibodies, so some people got over the infection before they made antibodies. In that case, antibody production can stop. But the vaccinations cause a much better immune response than mild infections.

Unless you have some condition that wipes out your immune system, or are taking immunosuppressives to wipe it out, you will respond. People who get infections after vaccination do not have no or poor response. They have a high viral load at the exposure. It takes a while to kill off all the virus, and then their immune system over-responds to dead virus fragments. The more virus you have to begin with, the more virus fragments are left floating around, and the worse symptoms. The immune system response takes a certain amount of time, so starting with a billion viruses leaves a lot more fragments than starting with a thousand.

That's one reason why mask wearing in public is so useful, even with a less than perfect mask: Lower viral load.

God sent Trump.......because God was out of locusts.