The software isn't scanned because it doesn't present a threat to the PC's functionality. As far as I know, these applications are rarely scanned unless the installation package is actually infected with malware.
You can use the QuickScan function in the application or the custom one to specify the install directory of the password manager.
In some cases, you can put the antivirus in an interactive mode which will ask you to scan everything once a program is installed.
