Documentation has been one of the pain points of my programming career. I've worked on some big projects with software that can't afford to fail and needs to me

Documentation has been one of the pain points of my programming career. I've worked on some big projects with software that can't afford to fail and needs to meet a bunch of requirements. All of them only had user manuals and zero documentation. A kind of documentation that I hate is documentation that just explains what a function does (adds x to y) instead of what business/real world reasons that logic exists (adds x to y because of "Z"). Explaining the why vs how is what documentation should be doing and I can understand why so many programmers shrug documentation when all they are doing is writing the how in non parse-able form instead of expressing why.