Several reasons for this... It's all about statistics, which most people do not understand at all. Human life span numbers includes all deaths at all ages, and the final number is an average. So, if you can eliminate a lot of infant mortality, childhood disease, and youthful accidents, the expected life span for an average person increases. There have always been lots of people achieving 80, 90, of 100 years, but if you can prevent lots of early deaths, more people live to old age, not necessarily to older age.

With pets, I don't think anyone really looks at or reports the entire statistical picture. When one looks up average ages that specific breed of dogs can expect to achieve, only the top end is reported - the mortality rate of puppies, abandoned dogs, or badly behaved ones who get euthanized, are never factored into the numbers.

I think the life span of many pets are lengthening. But it all depends on the breed of the animal and its environment.
It is unfortunate that today we have so many people involved in poor breeding practices which result in shorter lives of many (mostly popular breed) dogs.
You ask a good question, but it is very general and there are many many variables.....

Some of the improvement in human aging has resulted from improvements in technology in medicine - joint replacement, cancer treatment, heart surgery, etc. My two new knees cost over $100,000. These are great but very expensive. People often have some kind of insurance to help pay for it but there is a limit to what most folks can pay for a pet.

I have an 8 year old cat that I adore and want to keep around. I finally bought pet insurance for her - for catastrophic ailments, not routine stuff - so I could give her the best chance to recover from a serious illness. But, at about $30 a month it is not cheap and is out of reach for some.

Cooksnews explains it correctly. The potential for how long a human lives hasn't changed much, just more are reaching it. I was doing some research at the social security site and average life span has only increased about three years for males since its inception in the 1930s.

And right again with the explanation about dog/cat lifespans. That's the age a healthy, well-cared for pet might live. IOW the typical potential, excluding out of the ordinary longevity and premature deaths.

There is a parallel between human and pet health care. Both are becoming increasingly expensive, both don't really prolong the potential to reach a greater longevity, but allow a larger portion of the population to do so, and the statistics don't speak to morbidity in the aging population which of course is increasing along with the ability to treat conditions which used to claim lives at an earlier age.

Agree with Cooksnews. If you go to an old cemetery, you will find lots and lots of infants and young children. An old cemetery here has a separate area for them, and it isn't unusual to see several from the same family, buried next to each other. Those are included in the "average lifespan" figures, unfortunately. If a child then survived to age 6, he/she would likely live as long as we do. I have studied my father's family's geneology, going back several hundred years, and it isn't unusual to see people living into their 90's and beyond.

BTW, cancer is a major killer of dogs. There is more cancer in dogs these days than people. A couple of vets have told me this but aren't sure why.

There are some other reasons.

Nutritional advances contribute to extended human aging. We simply eat better.

Genetics is the biggest reason, however. Canines simply are not bred with longevity as a goal. The species evolved with a mortality much the same mortality as before man domesticated canines. We have developed many different breeds but never really included longevity in the breeding parameters.

Size makes a difference, although I do not have any theory/knowledge as to why. Large dogs live shorter lives than small dogs as a rule.