It's the unknown unknowns that really make estimates inaccurate. If someone were an expert in one exact thing and that's what they worked with day in and day out, then those unknowns would be small and the areas around them well defined.
It is relatively easy for someone to become a maintainer of one specific thing as long as that's the only thing they're doing for a long time.
Then you look at a software developer, sysadmin, anyone in the technical industries, and quite often it's not one specific thing, it's LOTS of specific things. Each with thousands, maybe millions, of human-hours in their development and the development of the things they depend upon. Every layer of hardware and software having it's own quirks and wrinkles.
It's like trying to be the god of a small solar-system of interacting planets each with plate tectonics and life-forms.
It is relatively easy for someone to become a maintainer of one specific thing as long as that's the only thing they're doing for a long time.
Then you look at a software developer, sysadmin, anyone in the technical industries, and quite often it's not one specific thing, it's LOTS of specific things. Each with thousands, maybe millions, of human-hours in their development and the development of the things they depend upon. Every layer of hardware and software having it's own quirks and wrinkles.
It's like trying to be the god of a small solar-system of interacting planets each with plate tectonics and life-forms.