It IS in some countries and states. Those that don't allow it generally don't allow it for one of two reasons:
1. Religion is against it. The average religion believes that your life isn't your own, it belongs to God. So it's not a matter of human rights - you have NO rights as a human to choose this.
2. More importantly, leaving that aside, where's the line between that and murder?
So where it is allowed, it's hedged around with all kinds of restrictions to protect anyone who helps out with euthanasia from a murder charge. So politicians fight shy of it, as it seems too close to making murder legal.
For example, the only place I know of where non-citizens can go and take advantage of the local law is Switzerland. The Dignitas clinic will do it for you, but to keep themselves in the clear, they will insist that you go there, talk it through thoroughly, ask you several times over your stay there if you're absolutely positively totally sure you want to die, and then when they're as sure you want it as you are, they provide a dose of a lethal drug but YOU have to drink it. It can't be euthanasia, it has to be assisted suicide. If you are unable to drink, they can provide a lethal injection but YOU have to push the button.
A country that does actually allow euthanasia is Belgium, and I mention this because of the story of Marieke Verwoort. She is a Paralympic athlete with a very painful disease which can only get worse. So she has signed all the legal papers to allow her doctor to give her euthanasia when she chooses, but as Dignitas has found, having done that is a kind of relief. Now she can live one day at a time knowing that it's all fixed up and she can choose her day. Maybe she never will. So far she hasn't. Her life can be one day at a time, maybe one day the pain will get too much, but so far not yet.
To quote Shakespeare, "tomorrow and tomorrow and tomorrow/creeps in this petty pace from day to day..." but the big thing for Marieke is she has the choice. She is in control. And paradoxically, the fact that she CAN choose to die has made it more likely that she won't! She could even make it to Tokyo 2020 - not to compete, but to enjoy being there.
So I totally agree with you, and thinking of Marieke, just knowing that the choice is there can be comforting. The difficulty is being absolutely legally sure it won't count as murder.