The environment is changing
Vaccination is changing the environment for the virus. And a change in the environment, leads to evolutionary pressure. The evolutionary pressure is going to be to resist the vaccine.
It hasn't happened yet. But it will. We've seen this with antibiotics - they slowly lose their ability to kill bacteria, because the bacteria evolve resistance.
Right now, there are two hundred million billion virus particles floating around (that's a quarter of a pint - they are really really small). And their only objective is to make more copies of themselves. But the copying process isn't quite perfect. Mistakes happen. Some mistakes mean that the copy isn't viable, some mistakes make the copy less infectious, some make it more infections. Evolutionary selection is for more infectious.
Some people don't believe that evolution happens. And some people believe that the world is flat.
The probability of a mutation that would evade the vaccines, is really really small. But there's a lot of virus particles, and the longer there's a lot, the more time there is for a vaccine-resistant mutation to occur.