Why does UAC still prompt me when "Run as Administrator" is permanently set?
This is by design. The checkbox negates the need to right-click and select "Run as administrator" every time.
Windows requires authentication for any task that requires administrator privilages. By what you suggest, nothing would prevent malware searching your computer for "Run as administrator" icons, running them, then using the application to access the depths of your file system. If the program ever displays so much as a file dialog, immediately the malware could modify files in restricted areas of the system.