Welcome to the Pain Management Forum. I am glad that you found us and took the time to ask a question.
Typically spinal surgery does not resolve 100% of a person's pain. Indeed ppl are often left with worse pain or different pain. However to the outsider it seems like if they had the problem "fixed" certainly there can be little or no pain. That simply is not the case.
Imagine the rearranging and trauma that your grandmother's spinal structure went through and then they placed a rod(s) in her spine. That has to be painful. I cannot say how much pain your grandmother is experiencing. I rather doubt she has become addicted and "likes" pain killers. I am more apt to believe that she truly has pain and the only "like" she gets from the opiates is that they control her pain and make her life better. Add her age and probable arthritic issues... as arthritis will more then likely begin if it wasn't there already and she has multiple reasons for chronic pain. Your attitude and doubt is fairly normal. Some ppl that have not been there have great difficultly understanding chronic pain, especially if they are younger.
A friend of mine (36) had titanium rods inserted a few years ago. He experiences significant pain everyday and requires opiates to help manage it. His nights are hellish and he has difficulty performing normal activities.
Your grandmother's weight may be compounding her pain issues but even at normal weight it is my guess she would still have pain. Her pain may keep her sitting much of the day. I can't judge as I haven't walked in her shoes. But I do know that ppl with chronic pain traditionally have a lot of difficultly obtaining family support and understanding. Family and friends often want them to "Buck Up" and get over it when it's simply not possible.
What your grandmother really needs is support and understanding. Chronic pain can be very lonely and isolating. My heart goes out to her. And to respond to your last line, I think this is a real medical issue.
The anonymous "Letter to People Without Chronic Pain" may help you understand a bit better from your grandmother's perspective. I hope you will take the time to read it. Here's the link:
I hope I have not been harsh, it truly was not my intent. I am just concerned that your grandmother may not have the support she needs. It's a terrible and lonely place to be at any age but sometimes even more so at 69.