> Couldn't nearly all of these attacks be blocked by a white-list approach, essentially hiding every file or directory from the internet except a very controlled list of paths and escaping all text sent so it can't contain code?

This is basically how things work.

For convenience, instead of itemizing each filename, the webserver root is a subdirectory and anything underneath is fair game. The webserver uses the OS "chroot" facility to enforce this restriction. What you are seeing is ancient exploitation strings from 30 years ago that haven't worked on any serious webserver since that time, but a) keeping the test in the attackers lib is essentially free, and b) there are some unserious webservers, typically in cheap consumer hardware.

Webservers pass plain text to the app server. It is the app server/framework's responsibility to understand the source of the request body and present it to the application in a clear way, possibly escaped. But the app needs to process this and sometimes through poor coding practices, fails to respect the untrusted nature of the data. This again is more typical in historical systems and low-cost consumer products where software is not a marketing advantage.