The user name contains different information from the password in that:
1. It is public, and some apps use the user ID instead of a real name.
2. It remains the same for a given account, unlike the password which ought to be changed on occassion.
3. In some apps, the data for the password will be encrypted such that even the sysop or administrator can't read it. This is the case for Unix passwords, and if you want to locate the user record, you need to look at something which isn't encrypted.