Freezing does kill bacteria, it’s just that it’s not so good at killing them that it can be considered a method of sterilization. It doesn’t kill ALL of them. I well remember when I was taking food microbiology, we visited a nearby factory that made frozen meals. They told us that sometimes, if a batch of meals tested slightly too high in some category of bacteria, they’d stick the batch in the freezer for a few months and then retest them, and usually they’d then pass.