This is just my personal opinion, but I think it's because the people deciding the rules of law are so far removed from the realities of the populations they lord over. All of these healthy, able-bodied, neurotypical politicians and lawmakers don't have the slightest clue as to what disabled people have to endure in our lives. Otherwise, they wouldn't be in politics most likely, as it is a demanding and gatekeeping career.
There are the virtue signalling campaigns here and there, false promises about progress and equality, but at the end of the day these people cannot put themselves in the shoes of a person who suffers day in and day out. They often are blind to the shortcomings of medicine, believing that if you just keep pushing, the healthcare system will eventually find a solution to everyone's problems. It would be great if this was true, but the reality is there are millions of people living with chronic illnesses, impairments, and disabilities who have been told we have to just deal with it the rest of our lives with no hope of reprieve.
Also, a lot of the disability related media that is put out for the world to see is inspirational in nature, you often see a disabled child with loving parents who would move mountains to support them. Or a sick adult who is provided for by their relatives and partner, so they don't have to worry about survival for the foreseeable future. They don't show what happens to disabled people when there is no longer anyone there to help you, especially if the disability is something more invisible to the naked eye in nature, or something which causes daily physical pain.
They don't tell the hard stories or the ones with no happy ending. I believe it's purposeful. It's easier to pretend everyone is happy and society is doing enough to ensure disabled people have good lives, than address the very real limitations of modern medicine and economic policy that force many sick and disabled people into poverty. It is inherently unpleasant to reflect upon this, that despite all the progress that's been made in recent decades, that life still is not anything close to fair and that our bodies are extremely fragile biological constructs subject to the laws of nature. We can only play god to a certain degree, with the aid of science and medicine.
I have had many similar experiences as you. Growing up with a developmental disability (Autism) people shunned me and my family was ashamed of me, but when I became suicidal as a child psychiatrists could not fathom that my disability would make me want to die. No, I must have had a hidden mental illness. Then my body began to fail me more and more, and I don't have a single day where I am not in some kind of physical pain or discomfort, suffering from 24/7 fatigue and lost so many of my cognitive abilities or anything that would make life worthwhile.
There are only so many times I can take being made fun of because I walk slowly, as my body does not have the energy to move and my legs are heavy and full of numbness all the time. Whenever I have tried to survive and provide for myself, no job wants to put up with a slow, disabled person, even though many times I am choking on the pain or fighting permanently blurred vision and brainfog due to a neurological condition. I am sorry to hear about your issues with mobility and dealing with pain, it's completely unfair that anyone has to grapple with those ailments.
People believe they are being compassionate when forbidding others from making decisions about our own life and death, but they are often ignorant about the toll that disability and ill health takes. My signature here (Xiǎng sǐ bùnéng - xiǎng huó bùnéng) means I want to die but I'm unable to. I want to live, but I'm unable to. It sums up my feelings about this topic pretty well.
I can't ever live any semblance of the life I want due to being so disabled and messed up, but I'm not 'allowed' to die either. The people who call the shots in this world are completely blind to the fact that there is the capacity to suffer for years without a light at the end of the tunnel. Unless it happens in their own personal life, they often stay blind to it. Many voters who have opposed euthanasia for the terminally ill don't change their mind until they are forced to confront the ethical dilemma themselves when a relative ends up in hospice care.