I mean, they are, that's what happens when you have a large portion of society vaccinated, but a large enough portion of society unvaccinated that it doesn't stop transmission. You've introduced a selective pressure that selects for variants that evade vaccine conferred immunity. Like when you kill roaches but don't get all of them and the survivors are more likely to have a genetic resistance to that poison. As soon as a variant mutates in a way that helps it infect the vaccinated, it'll start doing that, and over time it's basically inevitable eventually.