Why can't I "open command window here" inside a Library?

I'm talking about the extra options you get with left Shift + right click.

If you can open a cmd window elsewhere, then cd to a library directory, why can't you open the directory in a library?

I know that you can do some registry changes, to change this, but I'm just curious as to why this functionality doesn't exist in the first place.

You can add several locations to a library; the library forms a single consistent view over all these locations. Therefore, when you say open command prompt "here", potentially "here" actually refers to several places. I suppose Windows could open a command prompt in every one of them but that probably isn't helpful.

If you Shift+Right Click an actual folder inside a Library, you will get the option to open a command prompt. Similarly, if you expand the Library in the tree view and Shift-Right Click an actual folder the option exists.

There's a simple workaround for this.

Usually we do not need libraries, but we get trapped into one because the "Documents" command on Windows' main menu links to a library. In facts, if you access Documents this way you'll notice that in the address bar there's "Libraries\Documents".

Normally, however, we just need to access our very own Documents folder, which is normally named "C:\users\<username>\Documents". This is a single physical path, so the "Command Prompt Here" will pop up on Shift+RMB.

You can navigate to "C:\users\<username>" by Windows Explorer: there you'll find your physical "Documents" directory. Hodling Alt, drag and drop it onto your Desktop to create a link to it.

Next time (instead of selecting the "Documents" entry on Windows main menu) just double-click the previously created link, and here you go: you've just accessed a physical path. Welcome back, "Command Prompt Here"!