its not made to do this.
if you want to do it, heres how (i go through every single step since i don't know your level of experience):
-open My Computer >Tools > Folder Options... > View (tab at the top), under Hidden files and folders, change it to "Show hidden files and folders". click ok
-double click on the ipod drive, open iPod_Control. right click on the Music folder, select Copy (if you look at the music folder, it has a few folders named F00.... in side. in those folders are your music files in a somewhat random fashion (i think) with random filenames. you could just copy them from here, but the way i'm describing will be much easier and cleaner)
-Paste the music folder somewhere on your hard drive (on your desktop is fine)
-once it is done copying the folder, right click on the folder (the copy on your hard drive). Properties > Attributes, uncheck Hidden
-when you click Apply, it asks if you want to do just the folder, or the folder and all subfiles and folders. choose to do all of it. click ok
-open up iTunes. Edit > Preferences... > Advanced (tab) > General (tab) check "Copy files to iTunes music folder when adding to library". click ok.
-File > Add Folder to Library...
-find the Music folder you brought over from the ipod, and add the whole thing to the library.
-since you had itunes copy the files, you can delete the Music folder that is on your desktop. also, itunes will have organized and re-named all the files for you. you can go back and undo the "view hidden files" and "copy to library" options in my computer and itunes if you want.
and to answer your question of "why", apple is in bed with record companies to keep itunes running. they don't want to make it easy to copy the music files from one computer to another.
