Re: I'll bite :)
Because it's a matter of managing probabilities. Vaccination very substantially reduces the probability of catching a serious infection. It does not eliminate it. This is true of all the measures, all the social distancing, all the mask wearing, all the closure of premises. The lot.
My daughter, double vaccinated, still got an infection from her son, at that stage under the vaccination age here. It wasn't as bad as it might have been and will undoubtedly have increased her future immunity. She has still had a booster. Why? Because there's a new, more readily spread variant going about, resisting which needs all the strength the immune system can muster. And because, like me and SWMBO she's a bilogist, in her case a neuroscientist now working in clinical trials AND SHE UNDERSTANDS THIS STUFF.