Numbers perhaps. Family members would likely be unwilling to kill their own family members, even after they "turned". As a result many of them would become victims themselves.
Also, we don't know where everyone went as the show only looks at the small scale. So far, we haven't seen nearly enough zombies to account for the original population. There might be protected areas these survivors just haven't found yet.
Also, do zombies starve to "death" or do they just crave flesh?
I can't wait for the show to return.
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