A single user does not, metaphorically speaking, own the entire hard drive. Not only are each user's files constrained to a particular folder, but within that folder things also must be arranged a certain way. Applications and system files should not be moved at all. Some customization is possible for more advanced users, but the Mac veteran who creates new folders all over the hard drive is going to run into trouble fast. OS X does not subject its file structure and screen real estate to a single user's whims.
Users do get certain choices, however.
The screen can be scattered with folder and drive aliases if desired. More importantly, the appearance options, available fonts, items in the Dock, mail accounts and Internet favorites, among other things, are all kept separate for each user. Even a standalone user could benefit from this capacity for multiple work styles, and a workgroup of any size would certainly find it useful. It's simple to make each user's files secure through Sharing options, but by default they are visible to everyone. A desktop operating system is just about useless without applications.