I.e. "he is still smoking because he has a weak will"
The implied idea there is that he should try harder.
But my personal belief is that we have far less agency than we like to believe. That person with greater ability to say no to certain things (smoking, overeating etc.), can they take credit for superior moral fortitude? I don't think so. That ability to resit <insert vice> was hard-coded in their genes. They can no more take credit for it than a tall person can feel superior for being tall. You don't will yourself to be taller. It's written in your genes. Why would "willpower" be any different?
I'm not sure where that leads us though. Do we just throw our hands in the air and say "well, I'm subject to my genes therefore I'll stop trying". That doesn't seem very productive. In reality there probably is an element of hard work involved (although, the rabbit hole continues further. Why are some able to put in the hard work and others not? Genes again.), but there are limits to how much "stronger" you can make your will. Even though I will never beat Usain Bolt in a race I can improve my own PB through hard work, however my total ability is bounded by my genes.
It's a liberating view to take. Keep trying and don't fret too much when you fall short of whatever mark you'd ideally like to achieve.