As I see it, it is more nuanced. When engineer designs a bridge she does a very similar work to a programmer. Programing it is like designing a very complex bridge without constructing it really, because when you finished your design it had been built already.

So it is not exactly like you've said:

> You can't design it first and then go and build it.

You can design, but you cannot build.

A process of building by a design can be paralleled with deploying software -- suddenly there is a hairy real world, not all the hair was considered at the design phase, and either we hack around existing software (i.e. design plans), or call a programmer to redesign.