You can crash your car even while sober. And you can arrive home safely even while drunk. You don't have "complete control" over whether you crash. But the responsible thing to do is exercise what control you do have. Yes you could catch and spread the virus even after vaccinated but it is still irresponsible to disregard this and risk the lives and livelihoods of others. It is a trivial request and the consequences of spreading the virus are non-trivial.