Because (as mentioned above), 95% is not 100%. Since your in the flight business... If 100 flights were to come into your local airport and 95% of them landed safely.
And, of course, one other possibility is that it's a mutation variant that is able to slip through the existing vaccine. That's why even if you don't think you need it for yourself, do it for all the other people you come into contact with. We're all in this together and if you don't get vaccinated, you're potentially the Petri dish for the virus to mutate. If you want to get back to normal, get your vaccine!!