When did estimates turn into deadlines?

It's literally impossible to know what you don't know. Sometimes you discover things in the process of your work that couldn't have reasonably been accounted for on first estimate, and it is not good practice to always give an estimate that assumes a 5x difficulty multiplier for some unforeseen reason.

Yes, if this is an "every time experience" then there could be a skill issue or possibly some other undiagnosed or unrecognized systemic factor

When I cut open my ceiling to fix a plumbing issue I didn't know what I was going to find exactly, but I had a pretty good idea of the possibilities. My estimate for how long the fix would take was pretty close.

Software is the same thing. There are unknowns but there aren't unlimited possibilities.

It's a great analogy, but I think it's the kind of analogy that works on the surface and not in the details most of the time due to a) much more codification of practice in construction and b) more "fixed" knowledge of what's likely based on location and build era of the home, whereas software is much more dynamic and often subject to individual whims of developers or management.