The Ubuntu file browser does not show hidden files or folders by default, only the files that your user has access to.
Here’s what file browser looks like when browsing the root file system normally:
As you can see, all of the hidden/system folders are not shown. If you hit Ctrl+H at this screen, you will immediately see all the hidden files:
Fairly useful shortcut key. If you would just prefer to make this be the default setting when using file browser, go to the Edit menu and select Preferences:
Check the “Show hidden and backup files” checkbox, and you should now see the hidden files all of the time.