Seems weird that animal lifespans are so arbitrary. Goldfish and bears can live about 30 years, Parrots and Turtles can live about 90. So why do dogs and cats only live 10-15 years?
'cause some humans are stupid and can't look after their animals properly.
I don't see how it's arbitrary though. It's not as though once they hit a certain age they keel over...
I'm pretty sure that a dog not living to 90 has nothing to do with how well it's taken care of. And the arbitrary part is how the lifespans are all different.
That suggests some decision-making mechanism though, but I don't really want to get into that.
People do have an inordinate influence on how long a cat or dog lives though.
Uh, no. You can help or hurt pet lifespans to a certain degree with your control over their diet or exercise, but that's trivial in comparison to average lifespans, issues to do with specific breeds, &c. For example, large purebred dogs are rarely going to make it much past a decade due to a combination of inbreeding and health problems related to their sheer size.
Cats, however, are more in the 15-20 year range, assuming they're well-cared for (because no shit letting your cat run around outside in a city is an easy way to lose it). We've had five in my lifetime, none of them died younger than 15-16, and they all received similar care, but the actual lifespan was pretty arbitrary -- four of them were tortoiseshells and one was a Siamese, and of those, the toothless starving adult stray we took in lived the longest, while the other four were with us from kittenhood (the Siamese lived to be 23, the second oldest). Cats are also fairly predictable; if you don't let them out and risk car/animal/whatever killing them, it's going to be kidney failure every damn time. And there's nothing you can do about it.
tl;dr living conditions are not that significant. A dog or cat will not live to be 30 regardless of what you do, just as a human being will not live to be 200. All good care does is push them towards the high end of the natural spectrum.