Because they are smaller and have a short adolescence and a fast breeding cycle.<br />
In the wild, a cat, for example, can be fertile at six months, and completely grown at 1 year. That cat could have six kittens in a litter, and kittens tend to leave their mothers at 4 or 5 months of age. So, if a female cat has her first litter at 8 months, with an average of 2 surviving kittens per litter, and lives 8 years (typical for an animal surviving without human intervention) having a litter per year, she will have 16 successful offspring to carry her bloodline. Because of this fast breeding cycle, cats simply haven't needed long lives, so they haven't evolved them.
it is unfortunate, isn't it!
Obviously the advances Humans have made in medicine have a lot to do with it, but even without those advances we would still live longer than both cats and dogs. I guess I don't really know what causes some species to live longer than others. Good question.