Most well cared for dogs and cats are euthanized when they are at the natural end of their lifespans and no longer have a good quality of life. Bodies grow old and die. It doesn’t matter how much money we invest in medical care, human or animal. At best it can buy a few more (often painful) months or years.

Euthanasia is most often chosen when life is no longer enjoyable for the patient. I think the moral question the OP is posing is why we offer this kind relief to companion animals and not have this option for ourselves.