Why do most dogs have to be put to sleep, instead of dying?

My theory is that dogs have so much heart, they are indestructable.
My sweet old girl, Maggie is 16. I know most people have to put their animals down. I think they love us so much, they would drather be with us, even though it’s not easy. Mags is starting to glide gracefully down and has some trouble getting up. But she is so happy. She has been called the miracle dog.

Why do most dogs have to be put to sleep, instead of natural death? I know there is a pain and suffering factor. I want to keep my girl happy and healthy during old age, like a sr. citizen. She’s not to the point where I have to decide TODAY.

What do you think?

I have put my dear cat to sleep, she was 18 and very weak, with kidney failure.