It's just fiction, but you'd be amazed how much damage human teeth can do.

You won't be biting through bone (and idgaf what myth someone's heard about fingers and carrots and only stopping because your brain won't let you go hard enough), you won't even get through a joint in one bite.

But! We have enough bite force and sharpness to get into muscle and pull out pieces. Helps if you gnaw and use head movement to help, but we can even manage it with thicker skin than humans have. You'd be amazed what a drunk country boy will do to a dead animal if they have access to them before they're processed and there's a bet going. Not me, but I've seen someone go through cow hide and into meat. Wasn't easy, or fast, but it got done.

The real issue is the extent of decay involved. While most of the support for teeth isn't from anything fleshy, the process of decomp does loosen them slightly. It's pretty damn slightly though. The bigger issue is bite strength.

See, the kind of zombie matters, but the ones that look rotten would have weakened response from the muscles of the jaw and face, assuming actual decomp takes place. So, the kind you see on the walking dead and its spinoffs would eventually become ineffective, no matter how secure the teeth are in the jaw.

That being said, there's also examples in fiction, including the walking dead, of zombies lacking any muscles that would allow movement of a given body part, but still moving. There's scenes in the walking dead of zombies where you can see bone where muscle is supposed to be, moving arms in a way that would require the missing muscles. That isn't unique to TWD, but it's usually the easiest example to look up for the curious.

There's also non rotting zombies, more like the resident evil viral zombies. They may be mangled and/or damaged, but they're mostly intact, so biting would be perfectly possible in theory (individual zombies might have damage in the jaws that would make it impossible if real).

I can go on a lot more about how zombie fiction functions on a realistic level (or doesn't), but the generic attention span online has likely already been reached.