Hopefully you don't need surgery, but things happen and you might need a transfusion. Getting someone else's blood is always a little risky. And now you don't know if you're getting blood from someone who has been vaccinated. Perhaps you didn't want the jab, but in the event of a medical emergency, who knows what you'll be getting.

The American Red Cross reports that 20% of donated blood has COVID-19 antibodies

https://kyma.com/dsw-living/health/2021/03/15/covid-19-antibodies-present-in-about-1-in-5-blood-donations-from-unvaccinated-people-according-to-data-from-the-american-red-cross/