Because they are hidden/system files & you have “Display Hidden or System files” option turned off - which is the default windows setting.
That one little tick box prevents noobs from trashing their system & driving up M$ support costs.
If you prefer to maintain the option to trash your system at will, open my computer ->Tools->Folder options->View->
Turn on "Show hidden files & folders"
Turn off “Hide Protected operating system files”.
The following files should not be deleted:
Anything under C:\Windows, or C:\Program Files\Common Files.
Probably also not a good to delete anything under c:\documents & settings either, unless you know what you are doing.
Everything else is kind-of optional