> assuming you know what to build in the first place

This is the heart of the problem. Nobody knows exactly what to build. Most software development is half initial coding, half bug fixing, and 90% requirements discovery.

Compiler writers actually have it pretty easy once the language is defined, which is a real honest to god spec. Comparing how long it takes (and how much it costs) to write a compiler once the spec is done would be a pretty fair comparison to how long it takes to build a bridge once the design is finalized.

And even bridges can turn into total disasters. Look at the Bay Bridge replacement in San Francisco: https://en.wikipedia.org/wiki/San_Francisco%E2%80%93Oakland_...

Or the Big Dig in Boston: https://en.wikipedia.org/wiki/Big_Dig