Why aren't more people totally devoured?
It isn't explicitly stated on the show or in the comics, but the Max Brooks books World War Z and The Zombie Survival Guide suggest that zombies are very picky, and only eat fresh meat - either still-living, or killed within the last few minutes. This might be true of TWD as well. Furthermore, one zombie can't eat an entire person, even if it tried - you'd have to fit one person-sized amount of meat (minus some volume because large bones can't be eaten - they'd break the zombie's teeth) inside another person's stomach. This is impossible, because a human is much larger than the human stomach, or even the entire human abdomen.
Thus, people can't be totally devoured by one zombie, or even two or three zombies. A person can only be totally consumed by a large group of zombies, and even if there is such a group present, they usually don't finish eating the entire body.
We see this happen in Season 6, Episode 7, when a massive group of walkers attacks Nicholas, but eventually wanders off with much of his body, and his entire face, still intact. Later in the same episode, we see another Alexandrian, who had been trapped by hundreds of zombies, reanimated and mostly intact. This suggests that, as in Brooks' books, TWD zombies will stop eating when the body has been dead for long enough to be no longer "fresh", according to their standards.
As we saw in Season 4, natural causes - injury, old age, and disease - are much more deadly now than they were before the world fell apart. There are very few doctors left, access to medicine is extremely limited, there are no hospitals in operation (except rare exceptions like Grady Memorial, which is operating with drastically reduced supplies and resources, and has helped only a handful of people since the world essentially ended two years ago).
Getting sick is now a much more dangerous prospect than it had been when the miracles of modern medicine and technology were available. The survivors are only slightly better off than people were thousands of years ago, when a simple broken leg or case of the flu was a virtual death sentence. We're back to the days when 4/5 children died before their fifth birthday, and giving birth was highly likely to kill the mother, and often the baby as well.
Stress - emotional and physical - has a tremendous impact on the immune system, and the survivors are living under constant, intense stress, both emotionally and physically. The emotional stress is obvious - constantly fighting to survive, watching loved ones die left and right, and unrelenting uncertainty and fear - but the physical stress is probably more detrimental. Limited food resources, drastically reduced variety in the diet, limited access to drinking water, very little medicine - all of these things (which were slightly improved when the group reached Alexandria, but which are still a serious problem, even there) are taking their toll and making our protagonists less resistant to disease, and more susceptible to complications from injuries that we, in the pre-apocalyptic world, consider trivial.
Furthermore, the TWD Wikia points out:
The [zombie] pathogen itself does not kill its hosts, but it seems to weaken their immune systems considerably, to the point where even minor illnesses are far more fatal than normal to humans.
This is plausible, in light of what we've seen thus far on the show and in the comics, but had yet to be confirmed or explained.
In short, it is much more difficult to stay alive, even if we ignore the threat posed by zombies and hostile humans. The things that might make us, in the real world, mildly uncomfortable, are likely to become life-threatening situations for the survivors on the show.
The Kill Count Prior to Season Five:
There are statistics for onscreen zombie and human deaths on the show up to the end of season four. This site lists the kill count in seasons 1 through 4; I don't know if their numbers are exactly right, but assuming that they aren't wildly inaccurate, they are good enough for our purposes. I have taken the liberty of breaking down the data and creating a spreadsheet.
The members of the group who are listed by name in the chart have killed 956 zombies and 39 people. Other human characters have killed 229 zombies and 136 people. Zombies have killed 78 people.
The grand total:
1,185 zombies killed
253 people killed
In the comic books, and to a lesser extent on the show, Rick's group are depicted as being among the best fighters left in the world. They have repeatedly fought off much larger forces of zombies and bad guys alike. In comparison, people who aren't in Rick's group are usually zombie fodder.
The first four seasons of The Walking Dead represent perhaps 2 years of time. In 2 years, the best zombie killers around have only managed to kill 956 zombies. This does not bode well for humanity.
How Many People Versus How Many Zombies?
The blurb for the tenth issue of the comic book says:
After the ordeal Rick has endured last issue, he sets out to find safer shelter. More is learned about the zombies that now out-number us 5000 to 1, but when it comes to some things, it's better not to know. Rick begins to wonder if there is a light at the end of the tunnel his life has become. Even if there is, how can he ever expect to make it there?
In Issue #10, Carl is brought to Herschel's farm for the first time, having just been shot by Otis. This corresponds to the second episode of Season Two of the show. The second episode of Season Two takes place about a week after Rick wakes up in the hospital, and a few weeks since the outbreak began.
This means that within weeks of the beginning of the outbreak, only 0.02% of the population was still alive. How does this affect the data?
0.02% Humans, 99.98% Zombies:
Worldwide: 1,400,000 people against 6,998,600,000 zombies.
United States: 60,000 people against 299,940,000 zombies.
We're doomed, and trying to survive is just playing for time.