Or they don't know that the machine has been compromised as malware
silently installs and hides itself right in their face, because they run
as admin with full rights on the Internet with the machine, such as XP.
All of those MS botnet machines are not out there by accident, as the user
continually participates in the compromise of the machine, as they point
and click on everything under the Sun as user admin.
The bottom line is that there are millions of users out there and no one
including yourself can possibly know each and every one's knowledge and
skill level. There are many ignorant computer users, but even more with
more than adequate knowledge and skill sets.
Every safety device including UAC is designed with a good intension, but
when the user demands to return the control power, the basic design idea is
to return the full control power to the user, regardless of whether it's an
ignorant or skillful user.
UAC in Vista demonstrated a lack of basic understanding of providing a
meaningful and useful safety device which also is why for MS changed it with
Windows 7.
Whether you wish to accept it is your choice, but MS have accepted its won
design flaws.