The easy answer is that people want to die for all sorts of reasons that may or may not be a mentally sound reason.
Dogs want to live and know when it’s time.
I think a better answer is that people don't have the desire to control dogs as much as they have the desire to control other people.
That is a better answer. People are extremely lucrative, but only if they’re kept alive and able to spend.
Hmm, dare I say, if they cannot decide to end their lives on their own terms because such an option is illegal in most places... their existence is now purely to keep them alive, and that is not exactly cheap.
It’s not cheap, but it’s extremely profitable, hence why health care is so hellbent on keeping people alive as long as possible and precisely why euthanasia is so frowned upon.
So much of the health care expense comes at the tail end of life. Some people want the tubes and wires and the machine that goes 'bing'. The ones that don't should have a cleaner option than a shotgun or a noose.