That's because the OS developers have placed value on security over performance. Whooping cough is rare too, we still vaccinate against it.
If you want a classic example, bounds checking an array is important to avoid RCE and sandbox escape attempts. It can also have a hefty performance penalty, under some scenarios it trashes the branch predictor/instruction pipeline. But I'm glad that my browser isn't as fast as machine-ly possible when streaming video, because I'd prefer if there wasn't a risk of having my emails from various banks, stored passwords in the browser, etc from being collected and sent to a bad actor.