You can do quick prototyping without dirt in any language, it just turns out that you like working in dirt.

Honestly if I'm doing quick dirty prototyping I want to have as many tools provided for me as possible. I probably won't use most of them, but spinning up a quick Laravel project gives me built-in shortcuts for stuff that's just annoying to do in plain PHP. Sure there are conventions that say you should separate your code into different files, but if you want to you can just shove all of your code into the route files and ignore everything else.