Some labs made bacteria-free mice for research purposes. They're viable. So "not be able to live" is a stretch.

Human body does use some bacteria in the guts, but a lot of those very same bacteria would absolutely try to eat the entire body if its immune defenses ever failed.

Which is why the body uses those that it finds useful and absolutely murder those that it deems harmful.

It’s nature. Just nature. And it’s such a nonsensical discussion to have that the only reason for it to happen at any given time is to satisfy the ego of those saying “nuh-uh!!”.