Once you've launched a dozen sites (possibly for consulting clients or even old employers if you were careful with your contracts), you'll have a good set of tools built and lots of pieces that you can use to cobble together a new site in a hurry.

As an example, S3stat went from idea to launch in a weekend because the only thing I needed to build to make it work was the actual functionality of the product. The public site, its login system, user accounts, forums, CMS, admin backend, continuous build/deploy, and all the data access boilerplate were all torn out from other projects that I'd built over the years.

If I had to build all that stuff from scratch, it might have taken a couple months to have a good product site ready to compliment the comparatively tiny piece of code that is the actual functionality that people pay for.