I think it's because measurement, metrics and estimates are so hard in software engineering.
How long should it take to do X? If X is building a shelf, servicing a power plant or designing a car headlight I would imagine at least within the field the order of magnitude is pretty clear. Sure, better tools and experience may make someone twice or thrice as fast or the new guy is taking double the normal time, but such spans fit neatly into the human brain.
But now observe how neither of these statements seem off:
Getting my new web-based CRM tool written and published took me the whole afternoon.
Getting the new web-based CRM tool written and published took us 24 months.
Now add to that the fact that nobody really understands software top-to-bottom (or at all) and it is no surprise that everyone is confused all the time, builds weird mental models of things and skills while vaguely striving to achieve the first scenario and not the second.