Because it's reinstalling all the apps that actually takes time.

Yes, hopefully you put all the Really Important Stuff in your home or somewhere else where it can be trivially copied (and backed up...) - including your favorite editor, all your configuration files, and any other tools you can't live without.

But then you remember that you recently added a new tool to your workflow, a really shiny package, but one which also required installing approximately 1300 dependencies and for some reason a kernel driver. It took you a solid week to get running properly, and you've long forgotten all the hoops you had to jump through to do it.