Certainly part of it. Some people by the time they have no other hope are physically incapable of doing that.
Some people would rather not DIY such a consequential thing the same way they rather not brew their own antibiotics or do their own dentistry. Obviously in this case the “worst” which might happen is that they don’t die but suffer even more pain and indignities.
It is also the legal risk for those who remain. Every time someoen DIYs their end as you write it there is a police investigation. (As there should be, to make sure that there were no trickery around the death.) Depending on how things go your loved ones might get arrested and thrown into prison (if the system believes they killed you, or even if something they did or accused of doing is deemed to have “assisted” you). Similarly they might not be able to inherit after you if they were deemed to have assisted in your suicide. And that can be some small thing, like your partner driving you to the hardware store, or paying for the purchase, or helping you tighten a NATO connector. Now they lost you, they might have lost their home and they might be looking down the barrel of a risky legal case with years of prison as a possibility. That is not something anybody would wish on their loved ones.
I guess what you describe above sounds sensible. There are two practical problems with it.
One is that you are, by all evidence of your comment, an individual with above average inteligence and planning ability. I have a suspicion if we leave you on a deserted island in a few years you might be halfway on your way to your own iron age. Unfortunately not everyone who is smithen by devastating chronic suffering is as lucky as you are in that department. The rules of the society has to work for everyone, not just the smart ones. For each and everyone who thinks through what you did there are hundreds and hundreds who can’t and won’t.
Then the other problem, society wise, is that typically by the point someone reaches the end of the road and their illness is deemed incurrabble they are not well. Long illness weakened them, maybe they are paralysed, or blinded. What you describe is hard, but even harder if one does not have all their physical faculties. And that is very sad, but unfortunately that is the main case legislation like this aims at.
At least that is how I think about when i think about the purposes of such law.
If the law didn’t cover this, then people who provided assistance would be open to criminal prosecution.
That greatly depends on the jurisdiction. In general though the issue here is that you don't want to die by shooting yourself, but you want to die by taking a pill. And the issue really is that nobody is going to sell this stuff to you unless they know they cannot be charged for it.
Assisted suicide is legal in Austria for a few years and the main thing that it has changed is the availability of humane ways of dying.