And how would you want to distinguish those servers? Let's assume that you have several dozens of servers, each writing events via syslog to a central server from which you read those events from a file.
Your source is /var/log/whatever.log since it's read from a file. Your devices are dumb and cannot send FQDN as hostname so they only all send "router1" as name.
How would you go about deciding how many router1's are there?
It can be done in some particular cases, but in many cases - no.
Of course you can list sources or hosts from each index (that's trivial to do) but there's nothing except good practice and convention that would guarantee that such list would be complete and reliable.
