If they eat like they would eat im nature. You know chewing meat around hard bones, constantly grinding their teeth. Then it would be unnecessary. If you however just serve soft stuff or dog food there isn’t enough chewing and calculus starts to form if you don’t brush the teeth

Edit: this isn’t something I have source for. Just my own experiences with dogs and also human teeth need brushing for the same reason, we don’t eat what we were designed to eat. So it’s quite easy to make assumption that this is true for dogs too. And indeed when we fed our dog raw meat chunks with bone in it our dogs calculus went away pretty well and pretty fast