Because as much as the software industry loves to adopt the work "engineer", almost none of the sort of websites and software discussed here on HN ever gets anything much like "engineering" in the sense of, say, "civil engineering" done.
I think in my ~20year career, I've had only 3 projects that were specified well enough up front that we just "built it according to the plans" and had a satisfied customer at the end. Overwhelmingy, some (or most) of the design gets "made up as we go along". Even on projects with several small forests worth of up-front documentation, there's almost always large areas of vagueness or outright contradictory requirements, which need decision making on the developers part halfway into the job.
There are _very_ few websites that couldn't be coded from the ground up in a few weeks with a few good programmers - _IF_ you had already thought through all of the things the site needs and all of the consequences and the contradictions in those consequences. Even the big ugly complex projects, like Google's search or Facebook or Wikipedia or Twitter - if you had a spec that answered all the details about how you wanted them, a small team of experienced web guys could get it up and running in a month or so, and let you now how much cloud/hardware/sysadmin/support you'd need to budget for as your customer acquisition kicked in.
It doesn't take very long to "build" the software or the website (obvious "large scale" projects like OSes excepted). What takes most of the time is identifying and solving the problem. (And, I suspect a lot of the reason software/websites have a reputation for taking "so long" is that they're _way_ too often rushed into the "building" phase way before all the things that need designing are even identified, then all the new solutions take longer 'cause we try to fit them around all the work that's already been done.)