While smoking is a choice, not all choices are the same. Not all choices are weighted by a chemical addiction, comparing quitting smoking to bad food or exercise choices trivialises it in an unnecessary way.
Breaking an addiction is difficult, but smoking is one of the hardest addictions to break. I would imagine that heroin is more difficult to quit, but I can't think of any other addictions that are as difficult to break as nicotine.
If you have no experience of this (and it seems that you do not so apologies if you do) then a simple way to understand how difficult it is quit is not to think of it as a single choice to make. Imagine making the most difficult choice of your life, against everything that your body and brain is telling you - something that requires an extraordinary act of will and self-faith. The reward for making the correct choice is not an end to the matter - it will push it to one side for about 15-20 seconds. Then another wave of chemical addiction will wash over you, everything in your life will pale into insignificance and you will be faced with exactly the same choice again. This will repeat without end, asleep or awake, until you break the first part of the addiction. It will only take two or three weeks of these constant life changing choices, and you must make the correct choice every single time, but it will feel like an eternity.
If you make it through that initial period then it gets easier, but that sequence of life-changing choices won't really end for about six months, although you will gain more confidence that you are making the right choice through that time.
Several years later you will reach a point where you no longer have to choose and you have genuinely become a non-smoker. It is a long difficult road, but it is possible and many people do it. Believing that is almost impossible while you are still addicted, and find that believe that you personally are mentally strong enough to endure that cycle of choice and longing is the most difficult step of all.
So, yes it is a choice - but that does not make it the same as other things that are a choice. Some choices are more difficult than others, and to put all choices into a single category and assume they are the same is to under-estimate the range of difficulty that exists across different choices.