Yeah but an animal's ability to be physically active is seen as intrinsic to its quality of life. Same with humans to a large extent, but (unless in a coma) even when doped on painkillers in a hospital bed, grandma can still have moments of clarity, still have a story read to her or conversation. Of course, it should still be their choice, but i think it's revealing that so few of the terminally ill ever make use of legal assisted suicide. It seems to me humans are unique both in being able to enjoy at least moments of inactive living, and in knowing there will be no more of those moments when the plug is pulled
I think the real reason, though, to answer your question is 1) religion (few holy texts or preachers ever say "don't kill animals") and 2) it's relatively harder to pull the plug on your parents or children than on your pet and 3) long-term sickness is very profitable for hospitals, nursing homes, hospices etc. This last is the same reason the UK is coming up with cures to disease, while the US love affair with big pharma produces only symptom management